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  • Karma Houdini:
    • Lampshaded in the episode where Leonard meddles in Penny's date with Stuart.
    • Penny's ex boyfriend Kurt. In the Pilot, Kurt split up with Penny and got to keep their television set. When Leonard and Sheldon attempt to take the TV, Kurt strips them off their pants. He gets away with it. In the later episode "The Financial Permeability", Kurt is arrested for urinating on a police car. When arrested, it turns out he had a slew of unpaid tickets, as well as an outstanding bench warrant. Penny pays all his fines and Kurt does not intend to repay her. Months later, Leonard confronts him about this, and Kurt writes an "I owe her" note...on Leonard's forehead in indelible ink. Kurt does pay Penny back, but never reveals Leonard's involvement. So Penny is grateful to Kurt and they have a dinner date. Kurt faces no consequences for his behavior.
    • Wil Wheaton is often this. His debut appearance ends with him winning some money for pulling an underhanded sympathy ploy on Sheldon.
    • Sheldon is this in "The Weekend Vortex." He made plans with Amy to visit her family, and even though the Relationship Agreement requires her to do a ridiculous amount of things before he would agree such as 72 hours notice, checking the tire pressure on her car, and calling the CDC to see what kind of inoculations are required for Orange County (there aren't any). he decides not to go at the last minute instead opting to play video games all weekend. When Amy and Penny go to confront Sheldon, the episode turns into how lonely Raj is causing all the woman to leave and Sheldon to get off scot free.
    • Sheldon in general. He throws giant whiny tantrums to get his own way. Sometimes he gets Laser-Guided Karma for this, but sometimes it ends with the others giving in to shut him up.
    • After most of an episode with the guys angling to get into Wil Wheaton's Dungeons and Dragons game, he calls them out for using him to meet the celebrities he knows. The guys seem to genuinely like spending time with Wil, unlike the girls, who weasel their way into the game on finding out Joe Manganiello is in it. The episode ends on them happily sexually harassing him while the guys are frozen out.
  • Kids Prefer Boxes: Leonard jokes about this in a Season 6 episode.
    Penny: (referring to a package from Sheldon's mother) So what's in it?
    Leonard: Doesn't matter. Half the time he just ends up playing with the box.
  • Killed Offscreen: The character Debbie Wolowitz dies off screen and out of sight of her son Howard. She was visiting relatives in Florida. Howard receives a phone call alerting him of her death.
  • Killer Gorilla: The eponymous villain of the (fictional) B-Movie Serial Ape-ist is a sex-crazed killer gorilla. In his one known scene, the gorilla kills a nude blonde woman (played by Penny).
  • Killer Robot: "MONT-E" whom the guys built for a robot fighting competition.
    • Kripke's robot makes MONT-E look like a children's toy
  • The Killjoy: Sheldon is upset that the gang is having a get-together at Raj's place instead of his own so he makes a point of being as passive-aggresively annoying as possible.
    Raj: Okay, we've got fajitas with all the fixins, so you make your own.
    Sheldon: Wonderful. Dinner, some assembly required.
  • Kissing Cousins: Howard and his cousin Jeannie, apparently.
  • Kitsch Collection: The boys' compulsive accumulation of tacky sci-fi, fantasy and comic-book tie-ins could be viewed in this light. When Penny criticises Leonard for this and he is moved to sell his collection off to please her, Sheldon calls her about her accumulation of Care Bears, My Little Ponies, and other feminine-marketed kitsch. But the lonely Doctor Lorvis (who tries to romance Penny) tops everyone with his compensatory collection of high-end desirable kitsch - the Fortress of Solitude in the basement, that helps him escape being single and still living at home with his mother.
  • Kryptonite Factor: To resolve a running and escalating argument between Penny and Sheldon — Sheldon has just distributed her intimate underwear across telephone and power lines in the area — Leonard Hofstadter gives her the secret of Sheldon's personal weakness and the way she can utterly defeat him. He knocks on her door and gives her a deadly secret. It's the address and phone number of Sheldon's formidable mother.
    This is Sheldon's Kryptonite. You didn't get it from me.
  • Laser-Guided Karma: Raj invokes this when Leonard intentionally tries to screw up Stuart's date with Penny;
    Raj: Leonard pretends to be a friend to Stuart, but instead is a two-faced bitch. Thus, he is reincarnated as a banana slug!
  • Late-Arrival Spoiler: In trying to comfort Sheldon over a deception the others pulled on him while they spent three months at the North Pole, Penny referred to the '09 Star Trek film and explained a spoiler-heavy plot point as a comparison. Sheldon had not seen the movie yet. "I missed Comic-Con and the new Star Trek movie!"
    • In "The Spoiler Alert Segmentation": Sheldon spoils plot points from Harry Potter and the Half-Blood Prince and Deathly Hallows to Leonard who is currently reading Half-Blood Prince; Penny herself accidentally lets out a spoiler from Half-Blood Prince, more than SEVEN and FIVE years respectively after the books were released and a few years after the movies were released.
  • Laugh Track: The show, despite what the majority of people believe, does in fact employ a live Studio Audience, but like with other popular shows with similar setups, is that the audience ends up being filled by big fans excited to see their favorite characters live. This leads to a lot of laughter at the smallest of jokes, sometimes just because they know a punchline is around a corner note . And like other studio audience shows, it uses what is known as "sweetening" the laughter. Scenes that did not have the right reaction (where the audience laughed too loudly, drowning out the actors), too hard (ruining the comedic rhythm), too soon (preventing people from hearing the joke) or otherwise not reacting as to how or when they were supposed to (such as one scene where Penny kisses Leonard, the scene as-aired had no reaction, but the audience kept saying "oooh" on every take). If a scene doesn't work, however, the writers are not afraid to use alternate jokes thought up on the spot for a troublesome scene. In a number of Hilarious Outtakes you can see the actors sometimes reacting to the audience response. The closest the show comes to canned laughter is during outside portions (like Leonard and Sheldon walking down a street for example), they show the footage to the audience at the appropriate time during the taping, which does technically make it a Laugh Track to some degree.
  • Last Episode Theme Reprise : An acoustic version of the theme song is played at the last scene of the series finale.
  • Let's Get Dangerous!: Sheldon, gets a moment like this in the penultimate episode of Season 3. It's revealed that the reason Leonard puts up with Sheldon is due to the fact that before the series began, Leonard, Raj, and Howard were messing with some rocket fuel despite Sheldon's objections, which then threatened to explode. As Leonard got into the elevator to take the smoking fuel out of the building, Sheldon calmly entered the elevator, set the canister down on the floor and pushed Leonard out before closing the doors and leaving himself. When Leonard complained that he had plenty of time, the elevator doors exploded outward. Sheldon simply said "You're welcome" and walked back into the apartment. This is also why the elevator has not worked since the start of the show.
  • Like an Old Married Couple: Raj and Howard. Lampshaded by Penny and Sheldon in Season 5.
    • Heck, lampshaded by Leonard's mom somewhere in Season 2 or 3, when she called it an "ersatz homosexual relationship" created to fill the void left by their repeated failures at maintaining heterosexual ones.
    • Leonard and Sheldon are also occasionally portrayed this way.
  • LEGO Genetics: The Killer Gorilla in the (fictional) B-Movie Serial Ape-ist was genetically engineered.
  • Limited Social Circle: The four guys and the three girls. That's pretty much it. Even at Howard and Bernadette's wedding there were only two family members present, and evidently no other friends in the city who were worth inviting to the rooftop ceremony.
    • To be fair, the wedding was last minute. Compare the attendance of the wedding versus the attendance at Howard's bachelor party. There were at least 10 people in attendance for the stag party.
  • Limited Wardrobe: In a fun mix between this trope and Unlimited Wardrobe, everyone has a series of outfits that they rotate through but are largely on the same theme.
    • Sheldon generally wears two t-shirts, the outer shirt usually a superhero symbol like the Flash and the under shirt has long sleeves rolled up to his elbows.
    • Leonard has a jacket over a hoodie and brown pants he is always wearing. He often wears geeky t-shirt too, but are a little less flamboyant as Sheldon's (such as the chemical composition of caffeine).
    • Raj always wears a sweater-vest with a windbreaker jacket.
    • Howard wears uncomfortably tight turtlenecks under a button up shirt, odd colored jeans and wacky belt buckles. Oddly enough, among the guys he seems to be the most fashionably conscious and particular about what he wears, but it is still garish. He often wears a alien lapel pin on the turtleneck, which apparently has some meaning but won't ever be revealed.
    • Penny has a wider variety, but her leisure clothing also tends to repeat. In the early seasons she was known for wearing crop tops, tank tops and short shorts, by the third season she started wearing slacks and blouses more often, partly due to a leg injury Kaley Cuoco suffered that left her with a large scar.
    • Amy wears shapeless brown wool sweaters and long skirts, it's been lampshaded that her fashion sense seems to be 100 years old.
    • Bernadette wears button up sweaters and floral print dresses. The color pink tends to always be on her.
  • Lipstick Lesbian: Lakshmi.
  • Literally Loving Thy Neighbor: Penny and Leonard
  • Living in a Furniture Store: Downplayed with Leonard and Sheldon's apartment, it's immaculately clean but it fits their personality, especially Sheldon. But the walls and desks are lined with books, computer software and nerdy memorabilia, making it a lot more cluttered than most examples. Penny's apartment, on the other hand, is a lot messier and established as such early on in the series.
  • Loan Shark: Averted by Sheldon, to the cast's and viewers' surprise. Apparently, loaned money is the only thing which Sheldon is not a neurotic, schedule-obsessed jerkass about, if only because the only money he lends is money he doesn't need for anything in particular.
  • Local Hangout: The Cheesecake Factory, largely by virtue that Penny can be involved with the scene while she is working. The Comic Book Shop also comes up in later seasons.
  • Locked in a Freezer: One of Sheldon Cooper's more practical and sensible ideas is for the rest of The crew to acclimatise for a trip to the North Pole by spending time inside the lock-up deep freeze room at the Cheesecake Factory. Irked by a passing Sheldon insult, Penny invites him to reflect on the fact the freezer may only be opened from the outside and she might just go a bit deaf...
  • Logic Bomb: At least for Zack...
    Penny: I'm not talking to you!
    Zack: Then who are you talking to!? ...Babe?
  • Lonely Bachelor Pad: In a flashback to when Leonard became Sheldon's roommate, it's shown that the living room is furnished with just a couple of lawn chairs and a TV.
  • Loners Will Stay Alone: Subverted by Sheldon Cooper: he has no interest whatsoever in making friends and no redeeming personal qualities whatsoever, yet for some reason people keep talking to him.
  • Long-Distance Relationship: Leonard and Priya have one in Season 5. It ends in "The Good Guy Fluctuation" (Season 5, episode 7), when he learns that Priya cheated on him.
  • Long Game: Sheldon once knew Leonard hadn't returned a DVD to the video store, so he paid for the DVD and let it sit. It took seven years for Leonard to find it and try to make it right.
    • In Season Six, Amy mentioned that she has a five-year plan o get Sheldon to propose. He does. Five years later.
  • Long Runner: By series end, it will finish with 12 seasons and 279 episodes, marking it as the longest-running multi-camera sitcom in history.
  • Looking for Love in All the Wrong Places: Leonard, at the end of Season 3 and the better part of Season 4.
  • Loophole Abuse: Attempted by Sheldon in a game of "Magic Warlords of Ka'a". He plays a card called "Infinite Sheldon" that he claims beats all other cards and does not violate the no homemade cards rule because he "made it at work."
  • Loser Gets the Girl:
    • Leonard at Penny's Halloween party (though he ends up being Above the Influence).
    • From the same episode, Raj hooks up with a (admittedly) drunk girl. Specifically because he's too afraid to say anything.
      Drunk Girl: You're such a good listener!
  • Lovable Alpha Bitch: Penny. As a kid she was a straight up Alpha Bitch but has changed her ways (and even feels bad about how she acted before) She still has some bitchy qualities such as getting too greedy at times or being rather self centered, but is still a nice person for the most part.
  • Lovable Nerd: Leonard and Raj.
  • Low Count Gag: In "The Loobenfeld Decay", Penny's replacing a girl in a workshop production of RENT. It's a one night showcase, but they invite a lot of casting people and agents, so she hopes for a big break. She's not a good singer though, and there is a low count joke about the audience.
    Leonard: Why don’t you tell me about your showcase last night?
    Penny: Oh, it was okay I guess, wasn’t a big turn out but they both really seemed to like it.
    Leonard: There were only two people there?
    Penny: By the end. Yeah.

  • Made Out to Be a Jerkass: When Penny finally tells Howard that he sucks, he is so upset that he refuses to leave his room or help his friends with their robot for their upcoming battle with Kripke, so Leonard has Penny give an Ordered Apology. Then he ignores what she says and tries to kiss her.
  • Madonna–Whore Complex: Amy refers to an outfit she's wearing to a wedding with Leonard as "the perfect mix of Madonna and Whore".
  • Mad Scientist: Sheldon was once described as being "One lab accident away from being a super villain". Among other things, as a child he tried to order uranium to build a reactor, then attempted to build a sonic death ray when a government agent explained that he couldn't buy uranium. Lampshaded and referenced by Sheldon's childhood hero, Arthur Jefferies/Professor Proton (Bob Newhart) when he meets Sheldon.
    Professor Proton: Is uh, is he dangerous?
    Leonard: Actually he's a genius.
    Sheldon: I am.
    Professor Proton: That uh, that doesn't answer my question.
    • Wil Wheaton plays one in the (fictional) B-Movie Serial Ape-ist 2: Monkey See, Monkey Kill. His Love Interest is deceased and the scientist decides to clone her. Only her DNA gets mixed with that of the Killer Gorilla who killed her. Resulting in her coming back as a Beast Man.
  • The Main Characters Do Everything: While all of the characters have their own multiple interests, qualifying them as certain degrees of Omnidisciplinary Scientist, when it comes to their professional lives the show is actually very clear on what their specialties are. A long-form story arc has Leonard (experimental physics) devise a new idea of quantum mechanics, which he brings to Sheldon (theoretical physics) explicitly because he can't do the math that was needed. Howard (engineer) later suggests a gyroscope based on that idea, bringing all three together as Leonard and Howard work together on the mechanics of the device as Sheldon does the math needed to miniaturize it.
  • Male Gaze: Howard used a remote controlled camera to check out Penny's cleavage while Penny and Leonard were finishing up their first date.
  • Manchild: While all the characters indulge themselves in childish things, Sheldon insists that someone take care of him the way his mother would when he's sick (or even just homesick), locks himself in his bedroom where no one else is allowed when he's furious, curls up into a crying ball of sad on his bed when he's embarrassed, runs away from home when he's upset, is practically traumatized by the sound of people arguingnote , and is stubborn and petty beyond all reason. The only thing on this planet that can force him to behave rationally when he's angry or depressed is an order from his mother, who Lenard calls "Sheldon's Kryptonite". He also responds positively to being patronized by Penny, especially if he gets a toy robot and a comic book out of it. Lampshaded by Bernadette at one point, after she successfully got him to go to bed by first reasoning with him about the effects of lack of sleep, and then simply treating him like an overgrown child and sending him to bed.
  • Manic Pixie Dream Girl: In its own way, this is what Penny is to all of the guys and especially Leonard. The guys are generally happy in their own closed-off world of science, sci-fi, and comics, and Penny encourages them to expand their social horizons a little. In a reversal, Leonard (and Sheldon to a similar degree) have introduced Penny into a different world of higher thinking and education. In "The Ornithophobia Diffusion" Leonard and Penny argued over how they hated the things they do together (Leonard hated doing karaoke at 2AM or going on hikes, Penny hated watching a documentary on dams), only to later use those same things when talking to other people about their "hobbies" to make themselves seem more interesting.
    • Amy also seems to think Penny is this, ending up giving her a gaudy painting of the two as appreciation for "transforming" her into a "hipster who wears a bra that clasps in the front."
    • Howard's initial treatment of Bernadette would suggest he believed her to be this as well, when the truth is that she is often bringing him down to Earth. In another reversal, Bernadette has fended off questions about why she is with him by suggesting that Howards' energy makes life a little more exciting.
  • Master of the Mixed Message: Before Leonard openly admitted his feelings to Penny (she was well aware of his crush beforehand) she had given him several mouth kisses, once when drunk and once as a "happy birthday" thing. But it also shows this trope depends largely on interpretation as Leonard went into a mental maze analyzing Penny's reaction to a brief relationship he had with Leslie. He wasn't certain if it was "Genuinely happy you're with someone" or "Too bad you're taken now" and (post breakup) "Genuinely sorry it didn't work out" or "Glad you're now available."
  • Matricide: In "The Cooper Extraction", several What If? scenarios are depicted. In one of them an insane Howard preserves the mummified corpse of his mother. The cause of her death is not mentioned, but one of the interpretations mentioned by Bernadette is that Howard killed his own mother. Howard, the narrator of the scenario, does not care how his mother died as long as she is dead.
  • Meaningful Name:
  • Meanwhile Scene: In "The Stairway Implimentation", a flashback from Leonard contains one about what Sheldon does when Leonard, Raj and Howard left. Then lampshaded by Penny who notes Leonard couldn't have known what Sheldon did.
  • Meganekko: Bernadette, Amy and Leslie.
  • Meta Casting: Mayim Bialik, who plays Amy, actually has a PhD in Neuroscience (closely related to Amy's neurobiology). That makes her the only one in the cast who understands her dialogue. She even commented that the casting directors did a double-take when they saw that in her resume.
    • After the disaster that was the original pilot, Lorre and Prady decided to change the female lead into a more adorable, sweet girl who would actually be appealing to the audience. They cast Kaley Cuoco because her natural personality was exactly what they were looking for (in addition to her pre-existing credentials), in interviews the other members of the cast act significantly different from their characters except for Kaley.
  • Metaphorgotten: Sheldon, referring to Penny's singing.
    Sheldon: If cats could sing... they'd hate it too.
    • Penny to Sheldon, while the former was extremely high on pain killers:
    Penny: You know, people think you're this weird robot man who's so annoying all the time and you toootally are...but, then it's like that movie WALL•E, at the end! You're so full of love, and you can save a plant, and get fat people out of the floaty chairs...
  • Minor Flaw, Major Breakup: Leslie breaks off her brief relationship with Leonard when she finds out that, in her ongoing debate with Sheldon over whether loop quantum or string theory is the superior model of spacetime, Leonard doesn't actually care, and would have let their hypothetical children "decide for themselves" which one they thought was correct.
  • Minor Injury Overreaction: Inverted, on a date with Penny at the gun range Leonard shot himself in the foot. It turned out that it merely grazed his foot but Leonard tried to play it off as being badass that he had a gunshot wound.
  • Misblamed: In one episode Howard's mother has a heart attack after he tells her he's going to marry Bernadette. Howard thinks its because she was shocked about the news. They wait the hospital and eventually she wakes up and it turns out to have nothing to do with what Howard said. Bernadette is really mad at him for making her think that but when you think about it he really had no way of knowing (in Bernadette's defense she was likely really scared and stressed out about the whole thing.)
  • Misery Poker: Leonard, Raj, and Howard engage in this when Penny expresses disbelief that they haven't gone fishing with their fathers. Leonard and Raj's fathers may have been busy and scientific... but Howard's was missing. Leonard even says "Okay, Howard wins."
  • Mistaken for Gay:
    • Leonard and Sheldon often introduce themselves as "living together" and causing people to initially jump to the wrong conclusion, done before the opening credits of the pilot episode. Raj's parents, after hearing Sheldon say he lived with Leonard, replied that they were "like Haroon and Tanvir." Leonard then intervened and explained the actual situation, although later as Sheldon ran to get their special-edition DVD of Fiddler on the Roof he remarked to himself that "maybe we are like Haroon and Tanvir."
    • Sheldon also once went looking for an available male to distract Penny from an online gaming addiction. His technique was to approach a man in the cafeteria and ask if he was in a sexual relationship. Getting a no, he replied, "Would you like to be?" and walked away with the man's number scrawled on his hand.
    • Raj and Howard. In the third season, Leonard's mom encourages them to tell each other about their latent homosexual attraction, and tells Howard to stop talking about girls because it's upsetting to his "partner". Not to mention that Raj's parents say that Howard is the closest thing to a daughter-in-law that they have.
      • Leonard's mother calling Raj and Howard a homosexual couple is introduced in her first appearance on the show, the Season 2 episode "The Maternal Capacitance". Leonard loves the concept so much that he brings it up again in the above-mentioned third season episode, "The Maternal Congruence".
      • According to Raj's sister Priya his whole family thought he was gay
        Priya: My brother. He’s got a big crush on Bernadette.
        Leonard: What?! You’re kidding.
        Priya: No. I found poems he wrote about her. Very disturbing. “Oh, Bernadette, please play my clarinet.”... And for years, everyone in my family was convinced that HE was the clarinet enthusiast.
      • Lampshaded again in "The Cooper Extraction", an episode depicting What If? scenarios. Bernadette would never have ended up with Howard if not for Penny introducing them, because of Howard and Raj acting like a couple and Bernadette assuming they were.
  • Modesty Towel: Penny calls out for help because she slipped and fell in the shower. She's able to get the attention of one of the male characters, who she convinces to come in despite the fact she took a shower (and presumably is naked). He walks into the bathroom, and despite that she can't move because she broke her shoulder, she's lying on the bottom of the tub completely covered by a towel. Possibly she grabbed it when she slipped and pulled it down.
    • It's not a towel. It's the shower curtain. That still leaves the question of how she got it completely wrapped around her, though.
  • Moment Killer: Happens plenty of times between Leonard and Penny, and on occasion with Raj and Howard with their own prospective love interests.
  • Money, Dear Boy: In Universe, the way Leonard and Sheldon got Prof. Proton to come to their apartment.
    Leonard: Sheldon wrote him a check.
    Sheldon: Big check.
    • It should be pointed out that this is only in universe — reportedly Bob Newhart was and is a big fan of the show, enjoyed his time there, and has made several more appearances as Prof Proton.
  • Money Dumb: Raj is cushioned from the realities of life in the USA - until his rich father withdraws the financial cushion that has been bankrolling him. Straight away Raj runs into serious trouble - maxed-out credit cards are only part of it - and his friends question all manner of extravagances and fripperies when they attempt to balance his budget.
  • Mood Dissonance: Crops up here and there when the humor gets cringe-worthy enough, but the episode "The Staircase Implementation" deserves get a special mention. Leonard is just about to ask Sheldon why he shoved him out of the elevator when there is a huge explosion inside it. Everybody except Sheldon looks absolutely horrified and the Studio Audience... keeps... on... laughing...
    • When Howard finds an ALF doll, he reveals his mother gave him one when his father left and he pretended his father was taken to Alf's home planet. He asks the doll where his father was and is obviously just about to cry, yet the Studio Audience (or 'sweetened' Laugh Track) laughs through the entire scene.
  • Mood-Swinger: Part of the premise is that Penny is a very emotional girl who frequently baffles the guys with her illogical mood swings, shown in the pilot where she explains her life (from Nebraska, works at the Cheesecake Factory, etc) and then suddenly breaks down in tears (with Sheldon and Leonard looking at each other with a "Huh?" expression). At several instances she will storm into the apartment and yell at the guys for whatever recent mishap or screw-up, but she is also quick to forgive. Sheldon, being Sheldon, will sometimes associate it with "her time of the month."
  • Mood Whiplash: Season 5 featured a visit from Sheldon's mother, Mary Cooper. Mary preferred to sightsee while Sheldon wanted her to do "mom things" for him. At the end of the episode, Sheldon gets sick and his mother goes back to taking care of him. Sheldon sticks out his tongue at the group, knowing he's got his mom to care for him.
    • Another Season 5 episode wraps up with Leonard finding out Priya had cheated on him and while contemplating this, Sheldon scares the crap out of him by jumping out of the couch.
    • When Howard finds out about his mother's death
    • Sheldon's Meemaw finally comes to visit and he introduces her to Amy. She's the epitome of sweetness until he leaves the room to search her suitcase for a present. Then Meemaw lets Amy know she doesn't approve of her.
  • Moral Myopia: Both in and out of universe Sheldon expects (and is expected to) the people around him to bend over backwards for him, but doesn't see any reason to do anything to reciprocate. Many times he considers the bare minimum that he does for others (such as asking how Leonard's day went while admitting that he doesn't care) is equal to the unreasonable demands he puts on his friends, because he doesn't see them as equals. For example he got into an Escalating War with Penny over the rules at his apartment but also sees nothing wrong with imposing his rules at her apartment or anyplace for that matter. In addition it is bad to give Sheldon Spoilers but it’s perfectly okay for him to give Spoilers to others.
    • Sheldon also has no problem talking down to his peers or laughing at their mistakes yet gets incredibly depressed when the same thing happens to him.
    • Penny was quick to point out that for a bunch of guys who claim they were bullied growing up, they showed absolutely no reservation in making fun of her friendly but not-particularly-bright friend Zack.
  • Mortality Phobia: Sheldon plans to download his consciousness into a computer in order to live forever. When he gets concerned that the technology won't be available in his lifetime, he constructs a robot with a webcam and monitor so he can interact with others virtually while remaining sealed in his room away from anything that might harm him.
  • Ms. Fanservice: Penny. Her regular attire in the first two seasons was low cut tank tops and short shorts. Although it has gradually become averted as about mid-way through the second season she started showing less skin until the early third season where she hardly wears anything like that anymore unless going out to a party or dancing (although no less attractive).
    • After Kaley Cuoco's real-life horse-riding accident they stopped showing her legs to hide her surgical scars, which had no in-universe explanation.
  • Multiboobage: In "The Egg Salad Equivalency", Sheldon and Raj mention that Howard used University resources to build a six breasted sex robot.
    Howard: "Hey, I'm a feminist! I don't notice how many....breasts...a robot has...
  • The Multiverse: Frequently a Discussed Trope.
    Penny: What's a Multiverse?
    Sheldon : GET HER OUT OF HERE!
  • Mundane Made Awesome:
    • The sequence when Sheldon and Raj try to solve the physics calculation on Sheldon's board: A series of still shots of them staring at the board, from different angles, set to "The Eye of the Tiger." Played for laughs, of course. With an aspirin break in the middle, during which the music stops. It resumes when the staring does.
    • Pretty much every time the guys play video games together, you can expect them to be chewing the scenery into oblivion. "I possess the SWORD OF AZAROTH!"
  • Mundane Utility: It's a running gag for the characters using advanced scientific equipment for food-related purposes.
    • Leslie Winkle used a large laser to heat up a cup of noodles, and flash-freezing and smashing a banana to put over her Cheerios.
    • In "The Infestation Hypothesis" the guys use an expensive piece of lab equipment as a panini press.
    • Raj and Howard use a 3D printer to make lifelike action figures of themselves, and Bernadette. For about a piece.
    • Howard uses a robot hand designed for NASA to unbox their take-out food. It takes him a long time to program each movement, and they mention it took 28 minutes to do the entire bag. He later uses the robot hand for more... intimate... tasks.
  • Mushroom Samba: Howard gets some cookies from some old hippie women. They're laced with marijuana. He, Raj, and Leonard get high on the hash cookies and miss the meteor shower they went out to see.
  • Must Have Caffeine: Penny definitely seems to be this in the mornings. Drinking coffee while driving in the early morning is just one of the things she does that irritates Sheldon, and in the episode "The Vartabedian Conundrum", she comes into the apartment half-awake, saying desperately, "Out of coffee; need coffee."
  • Mutual Envy: Sheldon and Leonard, both brilliant scientists, envy each other their childhood. Sheldon's mum is too much of a religious nut for his taste (he still loves her), but he envies that Leonard's mother is rational, logical and a brilliant scientist herself. Leonard's mother is however extremely cold and emotionally abusive, and Leonard loves and envies how Sheldon's mother is sweet, caring and understanding.
  • My Beloved Smother:
  • My Friends... and Zoidberg:
    • Leslie's usual approach to greeting the four guys.
      Leslie: Hey, fellow scientists! ... (to Sheldon) Sheldon.
      Leslie: Hey guys. (to Sheldon) Dumbass.
    • When Sheldon's planning to move away, a section from his videotaped goodbye message to Leonard, Raj, Howard, and Penny: "The four of you are three of my closest friends, and one treasured acquaintance." The lowly "acquaintance" in question turns out to be Howard Wolowitz.
    • The fact that Sheldon, Leonard and Raj are all doctors and Howard technically isn't is pointed out by several characters, including Dr. Gablehauser, who pointedly addresses him as "Mister Wolowitz" and Sheldon, of course, who has referred to him as "Not-A-Doctor Howard Wolowitz."
      Wolowitz (miffed): I have a masters' degree!
      Dr. Gablehauser: Who doesn't?
      • When Sheldon gets a girlfriend who's a doctor and Wolowitz gets a girlfriend who's becoming a doctor.
        Penny: That means... Sheldon, you're a doctor. Leonard's a doctor. Amy's a doctor. Raj is a doctor. Bernadette is a doctor. And... Howard, you know a lot of doctors!
    • Sheldon actually sees himself as this to Wolowitz. He refers to Raj as Wolowitz's best friend, Leonard as his secondary friend and himself as his tertiary friend.
    • After Stan Lee's comic book signing:
      Leonard: Look at that: 'To my friend Leonard, from Stan Lee. Excelsior.'
      Howard: Awesome. Mine says, 'To my friend Howard, from Stan Lee. Excelsior.'
      Raj: Mine says, 'To Raj, from Stan Lee.'
  • My God, What Have I Done?: Howard almost says this word for word when he and Raj find the perfect match for Sheldon at the end of "The Lunar Excitation":
    Howard: Good God what have we done?
  • My Hovercraft Is Full of Eels: Sheldon's attempts to learn Mandarin.
    Sheldon: <There are oxen in my bed! Many, many oxen!>
    <Your monkey sleeps inside me.>
  • My Rule-Fu Is Stronger Than Yours: After years of putting up with Sheldon's ridiculously elaborate roommate agreement, Leonard starts dating a lawyer, who knows how to exploit every loophole in the language of the contract.
    • Played even straighter, when later Sheldon draws up a new roommate agreement that "benefits" himself greatly, then forces Leonard to sign it or expose them to Priya's parents.
  • My Sister Is Off-Limits!: Raj invokes this when his sister comes to visit. Not that it stops Leonard
    • Sheldon also invokes this when his sister visits, as Leonard points out that she carries the potential to bear children as smart as Sheldon. Not that is stops Raj and Howard from hitting on her in an extremely painful sequence:
      Howard: "Did you know the Jewish people invented circumcision? You're welcome."

  • Naked People Are Funny: Raj when he takes anti-anxiety pills that work a little too well.
    • Professor Rothman, after his mental breakdown, often shows up at the University in the buff.
    • On "The Parking Spot Escalation", Howard sits naked in Sheldon's spot, with Sheldon's laptop on his lap. Later, in retaliation, Sheldon gets naked in Howard's car.
  • Named After Somebody Famous:
  • Neat Freak: Sheldon. Has a need to impose regimented order in his own apartment order, Penny's apartment, and even Howard and Bernadette's closet.
  • Nerd Nanny: Penny. Serves as a caretaker to her nerdy friends.
  • Nerds Are Sexy: Pretty much the whole point of the show. The guys don't get laid despite their geekiness; they get laid because of it. (Except for the asexual Sheldon, for whom sex is less exciting than video games.)
    • In fact, the tagline of the show's first season was "Smart is the new Sexy".
  • Nerds Are Virgins: Played with. It's pointed out early on that everyone, even Raj (although Sheldon has no interest whatsoever), has had some intimate moments in the past. Their love life isn't exactly a feast though.
  • Nerds Speak Klingon: Practically a Running Gag.
    • "The Bad Fish Paradigm": Sheldon asks Penny a few questions about her compatibility with Leonard and one of them is "Do you speak Klingon?"
    • The episode, "The Panty Pinata Polarization" starts with the guys playing a game of Klingon Boggle.
      Sheldon: Excuse me, Penny, but we’re…
      Leonard: No, no, don’t tell her.
      Sheldon: …playing Klingon Boggle.
      Leonard: Aw!
      Howard: What do you mean, aw? Like she didn’t know we were nerds?
    • In "The Veracity Elasticity" Sheldon and Leonard use Klingon to prevent Amy and Penny from understanding them; the girls retaliate by conversing in Ubbi Dubbi.
    • "The Creepy Candy Coating Corollary": Howard says he speaks six languages if you count Klingon, Leonard and Penny agree that "Girls don't count Klingon". Later, after Sheldon wins a game in a Mystic Warlords of Ka'a tournament, he shouts at Wil Wheaton in Klingon.
      Wil: Did that guy just say 'Revenge is a dish best served cold' in Klingon?
      Stuart: I believe so.
      Wil: What is wrong with him?
      Stuart: Everyone has a different theory.
    • "The Vacation Solution" Penny thinks that Bernadette has talked Howard out of having their wedding invitations in Klingon, but Bernadette tells her that the Klingon text is on the back.
    • "The Hawking Excitation": Howard tells Sheldon no in several languages, including Klingon and Binary coded Ascii.
    • "The Countdown Reflection": Sheldon keeps trying to preform his part of the wedding ceremony in Klingon.
      Bernadette: Sheldon! I told you no Klingon!
    • In "The Romance Resonance", Howard sings to Bernadette and on of the things he lists that he would do without her is "speak a lot more Klingon".
    • "The Proton Transmogrification": While attending a funeral, Penny and Leonard both get pensive about dying and any regrets they might have.
      Penny: What would you regret?
      Leonard: Mm, you know, that I didn't travel more, take more risks, learn another language.
      Penny: You know Klingon.
      Leonard: That's true.
      Penny: No, I meant that as a regret.
  • Nerdy Bully: Dr. Barry Kripke, in addition to being incredibly nerdy, also checks off most typical high school bully tropes: he's crude, obnoxious, and routinely pranks the guys and foils their projects. Hell, he usually makes his appearances in the cafeteria like a high school bully.
    • There's also an episode in which Sheldon's World of Warcraft account is hacked, and when the guys track down the hacker he turns out to be a nerdy Fat Bastard much larger than they are.
  • Nested Story Reveal: Season 5, The Recombination Hypothesis
  • Never Lend to a Friend: Inverted, inasmuch as Sheldon honestly is not at all impatient to get back a sizable amount of money he lends Penny, but she misinterprets Sheldon's every innocent comment as a snide remark about her debt and his humorously tautological responses to her concerns freak her out. While explaining the true situation to her, Leonard remarks that it "the only one of his quirks that doesn't make you want to strangle him in his sleep".
  • Never My Fault:
    • When Sheldon receives a ticket for running a red light - issued by a traffic camera - he immediately blames Penny.
    • Penny also denies that it's her car, when Sheldon calls her out. She takes this attitude for quite a few of her traffic accidents.
    • Raj's one-time girlfriend Lucy in "The Romance Recalibration." She claims that their relationship didn't work out because he kept making her do things that made her uncomfortable. Ignoring the fact that Lucy kept ditching Raj on dates, didn't put any effort into fixing her personal problem, or making the relationship work. And she also forgets that the last time she went running back to him, he was seeing Emily, who was much more socially savvy, better looking, had a more interesting job and was genrally more fun to hang out with.
    • Another of his girlfriends, Deaf Emily (not to be confused with the Emily above) was a gold-digger. When Raj accepted being cut off by his father rather than drop the relationship, she unceremoniously dumped him once it was evident the expensive gifts he was giving her would stop. However when she returned in the episode above, Emily blamed the failure of their relationship on his being too reliant on his parents. The fact she was only interested in him because of his family's wealth never came up.
  • New Powers as the Plot Demands: Howard was never seen demonstrating his ability to communicate in American Sign Language until Season 5's "The Wiggly Finger Catalyst," although several times (including the pilot) he likes to make himself out as being a linguist (including knowing Klingon). It Makes Sense in Context because Howard acts as an interpreter between Raj and Raj's deaf girlfriend Emily in the episode, and as an in-joke that even with a woman Raj actually can date comfortably, Howard still has to be there.
  • New Media Are Evil: Averted notably in-universe whenever someone appears on National Public Radio: no one listens on an actual radio, but appear to listen to the live stream.
  • Nice Girl:
    • Mary Cooper, Sheldon's mother
    • Missy Cooper, Sheldon's twin sister
    • Meemaw (Constance), Sheldon's grandmother
    • Emily Sweeney, Raj's ex-girlfriend
  • Nice Guy:
    • Leonard Hofstadter.
    • Stuart.
    • Zack Johnson, one of Penny's exes. He is conspicuously thick but in refreshing contrast to the rest of Penny's exes doesn't seem to have a bad bone in his body.
    • James Earl Jones is definitely this, though he has a bit of a prankster side (as Carrie Fisher knows).
    • Wyatt, Penny's father
    • Alfred Hofstadter, Leonard's father
    • Randall, Penny's brother
    • George Cooper Jr., Sheldon's brother
  • Nice Job Breaking It, Hero!: Penny has a bad break up with an unseen boyfriend at the end of Season 1, and Leonard decides to go be a comforting friend. As he tries to make her feel better about the situation, he unwittingly convinces her that she should give the guy another chance. In a double layering of the trope, Penny's attempts to reconcile with the boyfriend fall apart (due to him already in bed with another girl) and she gets angry at Leonard for his "advice," leading Raj to say, "Wow! You managed to screw-up the screw-up!"
  • Nice Job Fixing It, Villain!: Priya cheating on Leonard in a Season 5 episode. If she didn't do that, Leonard and Penny would not have reunited later on.
  • Nightmare Fuel: Some disturbing stuff are acknowledged in-universe and they indeed invoked nightmares.
    • Leonard has a nightmare about Sheldon's reproduction and the mitosis method.
    • Sheldon himself has nightmares about Morlocks and Gollum, specifically himself turning into Gollum.
    • When Bernadette describes a real lab accident and then lies about it, the others look disturbed. As they should be.
    • Santa Claus fuels Sheldon's nightmares.
  • No Ending: Several episodes don't really resolve the story, and the next episode just carries on without acknowledging it.
    • "The Twenty-One Second Excitation" almost has a Bolivian Army Ending. In the final scene before The Tag, Sheldon and the guys make off with a print of Raiders of the Lost Ark, a horde of other nerds (including Evil Wil Wheaton and Captain Sweatpants) hot on their heels as the movie’s theme music plays. How is this resolved? Did they get away? Did they get trampled? Did Sheldon wind up in the hospital, or jail, or the loony bin? The Tag just cuts back to Penny, Bernadette, and Amy Farrah Fowler in Penny’s apartment. The episode ends there, and the plot is never revisited.
    • In the second season premiere, Leonard accidentally insulted Penny when discussing their relationship and the episode ended with her closing the door in his face. It was some 5 episodes later when we even learn that they had an uneasy agreement to stay friends and ignore their attempts at a romantic relationship, but the exact details of which are never seen.
  • No Guy Wants an Amazon: S4 Ep 05, "The Desperation Emanation", Bernadette sets Leonard up with her tough friend, Joy. Leonard ends up being miserable on their date due to her overbearing and shrill personality, including constant references to how strong she is and how much she's into fitness and martial arts (including 101 ways to rip a guy's nuts off). She also makes unwelcome comments about her bathroom habits. Although, when she implied she would put out he still agreed to be her date to a wedding.
  • No Indoor Voice: Mrs. Wolowitz.
  • No Man Should Have This Power: Parodied in "The Precious Fragmentation", where Leonard announces that he's mailed the prop One Ring back to Peter Jackson where it belongs. He's lying.
  • No OSHA Compliance: In "The Engagement Reaction" there is a patient in the hospital being treated with a very contagious disease. As it so happens the door doesn't have a latch and swings open freely, allowing Sheldon (who is desperate to avoid any sick people) to accidentally walk in without seeing the quarantine sign.
  • No Sense of Humor: Subverted by Sheldon; he's got one, it's just... very Sheldon.
    • "Bazinga!"
  • Non Sequitur: A Non Sequitur question. In Amy's Little House on the Prairie Fan Fic, Amelia loves Cooper but has trouble saying it. Leading to this: "All she longed to do was ask if his heart was beating as fast as hers, but she was too afraid to hear the answer. So instead she asked him in the future, if Montana would ever become a state...". This comes out of nowhere and is irrelevant to the story.
  • Noodle Incident:
    • Sheldon's childhood seems to be filled with these. Among the ones mentioned are Sheldon's dad fighting a bobcat over some liquorice, his parents arguing as his father trap shoots the family's commemorative plates off the roof, his mom's secret smoking habit, trying to build a nuclear reactor in his back yard, trying to frighten neighborhood bullies with a homemade Death Ray, being chased up a tree by a neighbor's chicken, getting beaten up by his sister, and losing an uncle who died chasing a badger up a chimney. Assuming Sheldon really Cannot Tell a Lie, it's a miracle he didn't turn out even more neurotic.
    • Amy's 'firm cervix' and her 'tilted uterus' was supposed to be next.
    • Penny was quoting her dad on how much he likes Leonard by comparing him to her rather peculiar collection of old boyfriends "Why can't you go out with Leonard? Leonard never tipped a cow on himself."
    • then there's this exchange:
      Howard: Close your eyes. Put out your hand. I got you something special.
      Bernadette: Come on, Howard, I'm not falling for that again.
    • How Sheldon got restraining orders filed against him by Leonard Nimoy and Carl Sagan.
    • Raj mentions an incident involving Sheldon getting food poisoning at the Rose Parade.
    • Howard tricks Penny into opening the door by using a voice recording to impersonate Sheldon. Penny mentions she wouldn't open the door for Howard after he gave her a teddy bear with a webcam hidden in it.
  • Not Distracted by the Sexy: Invoked by Penny for laughs on two occasions — one time, when the guys are discussing possible improvements to her entertainment system she makes the offhand comment that she would take all her clothes off because it's hot, and another time she brings three of her girlfriends to their apartment and offers to have sex with them while they are playing Halo 3. They don't even hear her.
    • Though, to be honest, what are the odds the girls would have made good on the offer if the guys had gone for it? Might as well carry on enjoying what they are doing.
  • Not Even Bothering with an Excuse:
    • One episode had Sheldon anxious at the prospect of giving a speech at an awards banquet. His friends offer to help him work through his anxiety in different ways. Leonard offers to do a therapy session with him, Raj says he will teach Sheldon some Indian meditation techniques, and Penny agrees to take him to buy a new suit for the occasion. Sheldon asks what contribution Howard will make, and Howard replies, "My contribution is to pretend I give a damn about your piddly-ass problems. And that's 24 hours a day, buddy." Sheldon appreciates his honesty.
    • When Raj has his date with Lalita, Sheldon excuses himself for being late, when asked what happened he just admits that he didn't want to be there.
    • Similarly, in "The Zazzy Substitution," Penny gets out of an awkward conversation at the Cheesecake Factory by saying, "Okay, I'm just gonna walk away, 'cause I don't want to be here."
    • In "The Lunar Excitation," Leonard asks Sheldon to move the nitrogen tank. Sheldon says he can't and when Leonard asks why, he says, "It's very heavy, and I don't want to."
  • Not Helping Your Case: Amy and Sheldon accused Leonard of being a sore loser after he "lost" an attempt at Counterfactuals. The game is very hard to win (unless you think like Sheldon and Amy), but Leonard's response didn't exactly do him any good...
    Sheldon: You should have been here for the Great Jenga Tantrum of 2008.
    Leonard: You bumped the table, and YOU KNOW IT!
  • Not So Different: Amy ended up looking through Leonard's yearbook and based on the comments noticed how unpopular he must have been, only to reveal that no one even signed her yearbook except her mother. This actually ended up as an ice breaker between them, who didn't really talk much before then.
    • Penny calls out the guys for making fun of Zack for not knowing much science in "The Justice League Recombination" when they were always bullied.
    • In "The Dead Hooker Juxtaposition", when a new neighbor threatens to replace Penny as the new Dungeon Masters Girlfriend, she complains that she's just manipulating the gang to do things for her, such as sending Howard to retrieve a TV from a relative. Sheldon notes that Penny did the exact same thing when they first met, sending Leonard and him to retrieve her TV from her ex-boyfriend.
      Penny: Apples and oranges, Sheldon!
      • To be fair, Penny enjoyed spending time with the boys from the start (and did things for them, like retrieving Sheldon's WoW stuff), whereas Alicia flat out states she is using them.
    • In "Higgs Boson Observation" Amy dragged Penny to the university to spy on Sheldon to make sure there isn't anything going on with his new (female) assistant Alex. While in his office Amy started licking his pencils and rubbing his stapler on her body to "spread her scent" and effectively mark her territory, which Penny expressed exasperation over. They eventually find that Amy shouldn't worry over Sheldon being attracted to Alex but do find Alex and Leonard having a friendly chat, which incites some jealousy in Penny. When Alex visited the guys apartment Penny acted far more possessive of Leonard than normal and eventually took him to bed, in effect doing the same basic "marking her territory" thing that Amy was doing.
    • Used yet again when Raj tagged along for girls night out. He mentioned offhand about how his love life hadn't been going well because he had a habit of crushing on the girlfriends of his friends like Penny and Bernadette, leaving Amy out of the loop. While hurt, she admitted that before meeting the group she knew what it's like to be left out and the two ended up bonding over the fact they were/are constantly rejected. By the end, Raj mused that she was suddenly much more attractive to him.
    • In "The Scavenger Vortex" Howard and Amy get teamed up for Raj's scavenger hunt and bond over their mutual love of Neil Diamond. It goes so far as the two of them abandoning the hunt to sing karaoke.
  • Not That Kind of Doctor:
    • In reference to Leonard's new girlfriend, an actual medical doctor.
      Penny: So who is she?
      Leonard: She's a doctor.
      Penny: Nice. A doctor doctor, or a you kind of doctor?
    • Also happens when Sheldon calls the police to help with his hacked World of Warcraft account, from "The Zarnecki Incursion":
      Police Officer: I'm sorry but I can't help you, Mr. Cooper—
      Sheldon: Doctor Cooper.
      Police Officer (looks over at Leonard incredulously): ...Seriously?
      Leonard: Not the kind with access to drugs.
  • N-Word Privileges: Invoked. Raj claims that when he refers to India as "Gandhiville", it's not racist.
    • Raj also calls out Penny for calling Jar Jar Binks stupid:
    We can say it, not you!"

  • Object Ceiling Cling: Howard's faulty space toilet malfunctions, and during their attempt to fix it, he launches a piece of meatloaf onto the apartment ceiling. It falls down some time later when Penny is there.
  • Oblivious to His Own Description: In "The Imitation Perturbation", Howard dresses up as Sheldon for Halloween and starts imitating him. Sheldon finds him annoying, but doesn't realize that Howard is supposed to be him until it's pointed out.
  • Odd Name Out: Given in-universe to one of his "clowder or glaring" of cats by Sheldon. While he named most of them after scientists in the Manhattan Project like Dr. Robert Oppenheimer, one was named Zazzles because, according to Sheldon, "he's just so zazzy".
  • Offscreen Breakup: Howard and Bernadette. They got back together, though.
  • Off-the-Shelf FX: In "The Cooper/Kripke Inversion", Howard scans Raj for his action figure with an Xbox 360 Kinect sensor.
  • Official Couple: Leonard and Penny. It was semi-official as of the pilot episode, and they were an actual couple for most of Season 3 before breaking up and reuniting in Season 5.
    • Sheldon and Amy.
    • Howard and Bernadette.
  • Oh, Crap!: Penny in "The Roommate Transmogrification" when she wakes up and discovers that she has slept with Raj. She actually didn't and they just fell asleep.
    • Penny again when she realizes that the wedding she had with Zack in Vegas wasn't fake.
    • Sheldon when he realizes that he told an FBI agent that Howard crashed the Mars Rover.
    • Sheldon again when he realizes Amy's plan to get him to associate happy memories with her presence is actually working.
  • Omnidisciplinary Scientist: Sheldon, a string theorist who on his days off also dabbles in, for example, genetic engineering, and who purportedly built a nuclear reactor and a "death ray" as a child, not to mention his extensive knowledge on the Social Sciences (History, Political Science, and Flags). This Depends On The Writer, though: we've also seen him unable to ascertain how to open a toolbox. Mainly, he just thinks he's an expert on everything; when he spends a day in Amy's lab, he thinks his intelligence qualifies him to do real neurobiology work, but can't correctly perform even the simplest tasks.
    • Truth in Television — anyone who's ever worked with extremely high-IQ academics will marvel at their often-paired lack of common sense and/or inability to perform mundane tasks. This is reflected in the reality that lower-level degrees are more general studies while higher level degrees tend to focus on specific specialties; thus two people could both have a doctorate in, say, electrical engineering and have little in common because one is for power-plants and the other is for microprocessors. Mayim Bialik is shown to be very particular in her phrasing when discussing Amy's neurobiology degree versus her own neuroscience degree because they are not the same thing.
      • “An expert is one who knows more and more about less and less until he knows absolutely everything about nothing.” ― Nicholas Murray Butler
    • This trope is generally subverted on other occasions, Howard (the engineer) boasts that he designs items for the international space station but he has no idea how to fix the elevator and when their car breaks down Leonard has this to ask:
      Leonard: Anyone here know how an internal combustion engine works?
      Raj, Howard and Sheldon: Of course!
      Leonard: Any know how to fix an internal combustion engine?
      Raj, Howard and Sheldon: Not a clue.
  • Once a Season: A couple of recurring gags you can expect every season: "Soft Kitty" being sung to or by Sheldon, Leonard and Sheldon's parents visit (although somewhat sporadically), everyone appears in costume for some event (Halloween party, Renaissance fair, the punishment for losing a game...) and at least one appearance by Wil Wheaton (from season three onwards) and Stephen Hawking (season five through eleven).
    • Each season also has at least one Leonard/Sheldon blow-out that only increases in intensity every year. The first season had an argument over the physics bowl, second season was Sheldon's interference with Leonard's relationship with Stephanie, the third season led to a Whole Episode Flashback with Leonard relating to Penny how they became roommates, the fourth season Priya started poking holes in the roommate agreement, the fifth season the roommate agreement was temporarily nullified and the sixth season Leonard started making plans to move in with Penny.
  • Once Done, Never Forgotten: The perennial joke of the series is that every character in the Fab Four is a doctor with exception of Howard. This gets compounded later in the series when his wife obtains her doctorate.
Leonard: "Doctor Gablehauser."
Gablehauser: "Doctor Hofsteader."
Raj: "Doctor Gablehauser."
Gablehauser: "Doctor Koothrapali."
Howard: "Doctor Gablehauser."
  • One Head Taller: Penny is generally about the same height as Leonard (whose short stature is frequently commented upon) but when she wears heels she suddenly towers over him.
    • Sheldon, being there is six inches between him and second tallest member of there group (and fifteen inches between him and the shortest) he towers over all the others. His twin sister Missy shares this physical trait with him, towering over all of the guys except her brother.
  • One-Person Birthday Party: In "The Cooper Extraction", several What If? scenarios are depucted. In one of them, Amy remains friendless and celebrates her birthday alone. She tearfuly sings the "Happy Birthday" song to herself:
    Happy birthday to me
    Happy birthday to me
    There are tears in the frosting
    Happy birthday to me.
  • One Steve Limit: Averted. There are two Angelas. One is Angela, a love interest for Raj, and the other is Angela Page, an FBI Agent. Both appear in Season 4.
    • In similar sounding names, there is Alicia the actress in Season 2 and Alice the comic book artist in Season 5.
    • Same name, slightly different spelling. There is Dmitri, a former physicist and current janitor in Season 1, and Dimitri Rezinov, a Russian cosmonaut in Seasons 5 and 6.
    • Raj has dated two girls named Emily. One was a deaf girl in Season 5. The other is Dr. Emily Sweeney in Seasons 7 and 8.
    • There are three Erics so far. One is Dr. Eric Gablehauser, the head of the Physics Department in Seasons 1 and 2. The second is Eric, who briefly dates Penny in Season 2. And the third is is Eric, a man who befriends Sheldon in Season 7, since they both love trains.
    • There have been four Georges so far. The first is George Cooper Sr., Sheldon's deceased father. The second is George Cooper Jr., Sheldon's brother. Then there are two guest stars playing fictionalized versions of themselves: Nobel laureate Dr. George Smoot in Season 2 and actor George Takei in Season 4.
    • Same name, different spelling. There is Glen, a mall security guard in Season 3, and Glenn, Bernadette's former college professor and boyfriend in Season 4.
    • In similar sounding names, there are three characters who sound alike. One is Joey Rostenkowski, Bernadette's brother. An unseen character. Second is Joy, a fitness fanatic dating Leonard in Season 4. The third is Joyce Kim, a North Korean spy who dated Leonard back in 2003.
    • Another set of similar sounding characters. One is Katie, the female lead of the Unaired Pilot. The other is an actress playing a fictionalized version of herself in Seasons 3 and 4, Katee Sackhoff.
    • Another set of similar named characters, five of them. There is Michael Hofstadter, Leonard's brother. An unseen character. Then there is Mikayla, a Las Vegas prostitute in Season 2. Then a trio of Mikes. One is Mike, who briefly dated Penny in Season 1. Second, there is Mike Massimino, Howard's NASA colleague in Seasons 5-8. Third, there is Mike Rostenkowski[, Bernadette's father. He appears in Seasons 5-7.
    • There are two Sanjays. One is Sanjay, a former boyfriend of Priya. The other is Sanjay, a cousin of Raj and Priya. Both are unseen characters.
  • Only One Finds It Fun: Sheldon Cooper has a knack for manipulating group activities so they end up skewed towards his interests, and once he's done that, no one else enjoys the activities, especially since he gets offended and tries to get petty revenge on them if they go against it.
  • Only One Name: Penny's last name has not been revealed in the show. This was revealed by the writers to be something of an accident, since the guys are (mostly) PhD's they were referred to as Dr. Cooper, Dr. Hofstader, etc. and their diplomas' are seen on the wall. With Penny they didn't even realize they hadn't revealed her last name yet, and by the third season they felt supersticious about doing so. Of course, by the ninth season she does have a known last name... Hofstader.
  • Only Sane Man: Mostly Leonard. Since his fixation on/relationship with Penny drifted away from center stage, this has become a big part of his role.
    Sheldon: I'm afraid I can't allow that. Pursuant to Starfleet General Order 104, Section A, you are deemed unfit and I am relieving you of command.
    Leonard: Starfleet General Order 104, Section A does not apply in this situation.
    Sheldon: Give me one good reason why not.
    Leonard: Because THIS IS NOT STAR TREK!
    • Penny possibly counts too, especially compared to Amy and maybe Bernadette.
    • Subverted in "The Love Car Displacement" during the science panel, when Sheldon of all people is (briefly) the Only Sane Man.
  • On the Rebound:
    • Leonard's attempt at a "serious" relationship with Leslie was this, after his first time attempting to date Penny went south. At one point he and Penny run into each other across and the hall with their new romantic partners. They swoop their respective into a passionate kiss and try not to stop until the other does (which goes on for at least a few minutes) in an effort to show just how "fine" they're doing without each other.
    • After Leonard and Penny tried dating the second time Penny gets together with Zach. She admits that while he's a nice guy, he's dumb as a sack of hammers and the only reason she bothered to give him the time of day was so she wouldn't be lonely and to take her mind off of Leonard.
  • Operator from India: Invoked Trope, When the gang starts up a software development company Raj is offended that Sheldon's assigned him a job in customer service; Sheldon explains that it's in order to give the impression that they're a large multinational.
    Raj: "Ohhh, very clever. But still racist."
    • In another episode, Raj suggests that he would put on an American accent when dealing with tech support because he doesn't want them thinking he was making fun of them.
  • Ordered Apology: Often, and Sheldon is usually involved. Sheldon's mother tends to be brought in to force him into apologizing, and Missy deals with his unapologetic stubbornness through Groin Attack quite effectively as well.
  • Orphaned Punchline: "So anyway, to make a long story short, turns out I have an unusually firm cervix"
  • Outdated Outfit: Leonard's collection of ugly suits, including a felt maroon-ish one. Sheldon also has a suit more appropriate for the 1930's.
  • Out-Gambitted: Sheldon tries the old "joy-buzzer" prank on Howard by getting Howard to shake Sheldon's hand with the buzzer attached. Howard collapses and Bernadette is immediately upset, reminding Sheldon that Howard has a heart condition. Bernadette pulls out an adrenaline needle and insists that Sheldon inject it into Howard's heart, claiming not to be strong enough to do so herself. Howard opens his eyes and reveals the ruse. This one seems like it had to have taken some planning and anticipation on Howard and Bernadette's part for whatever Sheldon had in store, but it may have been easier if you factor in Howard's familiarity with The Sheldon as demonstrated in the above Improbably Predictable example.
    • It gets even better when Sheldon smacks himself in the forehead for falling for it and gets a dose of his own joy-buzzer.
    • In the same episode, Sheldon apparently rigged a prank in the mail box and was very obviously trying to lead Leonard to it. Leonard figured out the prank beforehand, and rigged it so it was on remote control and would go off when Sheldon opened the box and was greeted with a blowup pillow with Leonard's face on it. This pretty much sums it up
      Leonard: You may be from Texas but I'M from Jersey
  • Out-of-Character Moment: Happens to Penny in the episode "The Dead Hooker Juxtaposition". When Alicia (a new, pretty girl) moved upstairs and was manipulating the guys with her looks, Penny had a conversation with her in the laundry room. She tried to explain the idea that they don't know how to put their shields up...
    Penny: You know, in Star Trek when they go into battle they put their shields up... (realizes what she just said) ...Where the hell did THAT come from?
    • Happens to Sheldon in the episode "The Adhesive Duck Deficiency". This fuels the comedy when Sheldon is the only one to help Penny when she dislocates her shoulder in the shower. Penny asked him to try to be "comforting" and Sheldon's attempt to be comforting is so alien that Penny is visibly disturbed by it.
    • Happens to Howard in the episode "The Financial Permeability". He and the other guys attempt to confront Kurt about the money he owes to Penny. Howard is afraid of what might Kurt do to them. So he makes the sign of the cross for protection. But Howard is not a Christian but a Jew.
  • Overcomplicated Menu Order: A Running Gag, with Sheldon being very specific on how he wants his food prepared. It was also done once with Leonard's mother.
    • In the episode "The Einstein Approximation", where Sheldon decides to work at the Cheesecake Factory in order to let his mind work on a physics problem, Leonard gives him an extra-complicated order in revenge for all the times Sheldon did it to him.
  • Overly Long Hug: The guys are in a position for an expedition to the North Pole and when Sheldon informs Penny of the trip, she shows unusual concern about Leonard leaving for several months. Later, she privately gives him a going-away present (a blanket with sleeves) and a surprising hug that goes on a lot longer than Leonard expects, showing him trying to break away sooner than her. All things considered, the hug is the main sign to Leonard that she is developing romantic feelings for him. When Leonard approaches her about it, he says "It was at least 5 Mississippi."
  • Overprotective Dad: Intentionally subverted/invoked with Penny's father, who really likes Leonard and considers him far above the dumb, brawny jocks she usually dates. When he discovered they actually didn't get back together, he implies he's going to verbally assault Leonard, only to quietly beg him to not give up on her. He then tries to use reverse psychology on Penny (who was in her bedroom) by yelling at Leonard to never see her again.

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