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.
Sometimes though, when they want to show Sheldon sympathetic in episodes like "The Guitarist Amplification" or "The Bozeman Reaction" the laugh track pretty much hinders this.
It also leads to no end of people complaining about the laugh track and how fake it is.
Let's Get Dangerous: Sheldon, gets a moment like this in the penultimate episode of season three. 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.
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.
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. Sheldon has a Flash t-shirt that shows up often, Leonard has a jacket over a hoodie and brown pants he is always wearing, Raj is usually wearing a sweater-vest and Howard wearing uncomfortably tight turtlenecks and odd colored jeans. Penny has a wider variety, but her leisure clothing also tends to repeat.
Averted with Howard, who has a particular style of wardrobe, but a massive number of tight turlenecks, coordinating shirts, too-tight pants, and wacky belts.
Howard also uses "cheater turtlenecks", or a piece of clothing that goes around the neck, but ends at about the pecs. Easily visible when he rips off his shirt to swing above his head in the Physics Bowl.
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...
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 exlained that he couldn't buy uranium. Lampshaded and referenced by Sledon'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.
Male Gaze: Howard used a remote controlled camera to check out Penny's cleavage while Penny and Leonard were finishing up their first date.
Man Child: 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 Although this one may be perfectly justified, as his parents often got into arguments when he was a child and it is also implied that a lot of the arguments were exceedingly violent (he mentioned that his father would start throwing plates and shattering them in the kitchen, and that his mom planned to place glass shards in his father's meatloaf), even when they said to Sheldon that they stopped fighting, 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."
Mata Hari: Joyce Kim, as portrayed in "The Staircase Implementation".
Interestingly enough, her name implies brightness — i.e. "bright as a penny". This may be the creator's way of implying that she, as she has actually shown in many episodes, has more going on up there than many give her credit for.
His last name also sounds like a combination of "wallow" and "wits." He wallows in his own wit, as it were — or he would, if Sheldon didn't constantly deflate him.
The head of the Entomology Department at the University is Professor Crawley, he mentioned he had the nickname of "Creepy Crawley" in school.
Mary and Shelly Cooper have to be a shout out to the writer of Frankenstein.
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.
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.
Sheldon: If cats could sing... they'd hate it too.
Milestone Celebration: Celebrated the 100th episode in the fifth season with an appropriately big episode.
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.
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."
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". 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.
The empty rings on the bar above suggest that Penny yanked off the curtain to cover herself before calling for help.
Or that she grabbed it when she slipped and pulled it down.
Moment Killer: Happens plenty of times between Leonard and Penny, and on occassion 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 a second appearance as Prof Proton, looking towards a possible third episode.
Mood Dissonance: Crops up here and there when the humour 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...
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.
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 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: Leslie, such as using a large laser to heat up a cup of noodles, or flash-freezing and smashing a banana to put over 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 $3000 a piece.
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.
Leslie's usual approach to greeting the four guys.
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 says this when he and Raj find the perfect match for Sheldon at the end of "The Lunar Excitation".
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.
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."
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.
Never Lend to a Friend: Sheldon honestly doesn't care about the money he lends Penny, but she gets neurotic about it, misinterpreting Sheldon's every innocent comment as a snide remark about her debt.
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.
Nice Job Breaking It, Hero: Penny has a bad break up with an unseen boyfriend at the end of season one, and Leonard decideds 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 five 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.[invoked]
Sheldon himself has nightmares about Morlocks and Gollum, specifically himself turning into Gollum.[invoked]
When Bernadette describes a real lab accident and then lies about it, the others look disturbed. As they should be. [invoked]
Santa Claus fuels Sheldon's nightmares. [invoked]
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 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 Ep05, "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 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 accidently walk in without seeing the quarantine sign.
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."
And the winner:
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 inncedent involving Sheldon getting food poisoning at the Rose Parade.
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.
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.
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 neighbour threatens to replaced 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!
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.
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.
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.
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".
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 three.
Sheldon and Amy.
Oh Crap: Penny in "The Roommate Transmogrification" when she wakes up and discovers that she has had sex with Raj.
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) as well as basic Notary Law (he is the principal author of the Roommate Agreement with Leonard and the Relationship Agreement with Amy). This Depends On The Writer, though: we've also seen him unable to ascertain how to open a toolbox.
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.
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 from the third season on, at least one appearance by Wil Wheaton.
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 lead 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.
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.
Only Child Syndrome: Very much averted. Howard is the only main character in the series who is explicitly stated to be an only child. Sheldon has an older brother, George, and a twin sister, Missy; Leonard has an unnamed older sister and a younger brother, Michael; Penny has an unnamed brother and sister; and Raj has a younger sister, Priya, three unnamed brothers, and another unnamed sister. Bernadette, Amy, and Stuart have an unspecified number of siblings as well. Numerous aunts, uncles, cousins, nieces, nephews, and grandparents are also mentioned throughout the series.
Only One Name: Penny's last name has not been revealed in the show.
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.
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.
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.
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-of-Character Moment: When 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?
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.
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 familiarty 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
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.