Maybe you claim to be from the future, or maybe you say you are some reincarnation. Whatever it may be, Mr. Suspicious isn't buying it. So what do you do? You tell or show him the one thing that absolutely proves your claim beyond a shadow of a doubt.
A subtrope of God Test
. Compare Something Only They Would Say
and Only the Knowledgable May Pass
. May be in Spy Speak
if it's a sign/countersign challenge.
characters naturally have at least one of these.
Not at all related to Safe Word
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- A Tim Horton's commercial had two Canadians crossing the border, with proof of identity given by the "R-R-Roll up the R-R-Rim to Win" slogan. The next two in line mispronounced it.
Anime and Manga
- In the fourth Haruhi Suzumiya novel, Kyon finds himself in an alternate universe where the SOS members are ordinary humans. Haruhi won't even give him the time of day until he desperately introduces himself as John Smith, the alias he had used as a time traveller helping her out three years ago. Once the world is returned to 'normal', he keeps this as a trump card (emergency use only!) to blackmail Yuki's boss.
- Previously played straight in Bamboo Leaf Rhapsody: after Kyon hands over to Yuki the paper piece she will give him in three years time (he's time traveling at the moment) she is convinced enough to synchronize (sharing memories with her future self, effectively making her current self and Yuki in three years time the same person). Inverted immediately after when Yuki takes her glasses off, showing a habit she will take by Kyon's request in three years time.
- Played with multiple times in a different context. Adult Mikuru tries to use a distinctive mole on one breast to convince Kyon of who she is the first time they meet, only to realize that Kyon hasn't actually seen the mole on Younger Mikuru yet. Much later, Kyon uses the lack of a mole to realize that the Mikuru in his room is a fake.
- Probably worth mentioning that Kyon then proceeded to tell Younger Mikuru about the mole, which she didn't know about yet, which is how she finds out about it in the first place, but that's a different trope.
- And in Disappearance, attempting to use this Trust Password on an alternate universe Mikuru utterly fails, and gets him labeled a pervert and punched out by Mikuru, of all people.
- Yusuke of YuYu Hakusho regrets not having one of these when he possesses Kuwabara, and not a single person believes he's actually Yusuke.
- He ends up identifying himself to Keiko by grabbing her breasts (or, in the edited-for-TV version, peeking up her skirt), which elicits a reflex response of "Yusuke you jerk!" and a slap, at which point she finally accepts that it's him.
- And that it was unnecessary because she recognizes his body language. Oh well.
- Played With in Naruto. Sasuke Uchiha, having just defeated an imposter through recognizing him by luck, gives one to both Sakura and Naruto to say. However, he knew that the real Naruto wouldn't be able to remember it, and (correctly) identifies another imposter based on this several minutes later.
- In a filler episode of Ghost in the Shell: Stand Alone Complex, a terrorist known as "Angel's Wing" ("Angel Feathers" in the English dub) gives one of these to his blind, wheelchair-bound daughter so that she'll know it's him even if she doesn't recognize him. At the end of the episode, Angel's Wing is captured in a chapel... Right before his daughter enters. She then says her half of the password ("What is the angel planning to do today?") At first, it seems as if Batou, who had heard both halves of the password earlier, is going to say the second half (something like "The angel will rain feathers down from the heavens") if only to make the girl happy, but he doesn't, changing it to ("the angel... isn't planning to go anywhere").
- One of the final episodes in 2nd Gig has the Big Bad trying to send Rangers after Section 9. They immediately stand down after running into Batou, a former Ranger; something they can immediately tell because all Rangers have the exact same Electronic Eyes.
- In One Piece, after learning that one of the villain's lackeys can duplicate appearances, Zoro comes up with a solution; To distinguish each other, each Straw Hat should wear a white cloth over their arm to hide the true trust sign...an X.
- During the The Death of Superman storyline, the real Superman proves himself to Lois with Clark Kent's favorite movie: To Kill a Mockingbird.
- In later comics, Superman's codeword to Lois is "Beef bourguignon with ketchup". Though in this case, it's a way to let Lois know through an intermediary that he's safe. A farming tradition. Subverted when Parasite poses as Lois and is able to use the password.
- During the Obsidian Age arc in the Justice League of America, in which the entire League is killed and their souls trapped by an ancient Atlantean sorceress, it's revealed that Batman's passcode for Nightwing is "The Daring Young Man on the Flying Trapeze"; a reference of course to Dick Grayson's boyhood career. Also, song lyrics.
- Knights of the Dinner Table
- Inverted by Genre Savvy players in the comics. Often, players will set up a "Doppleganger Password"; should a character become separated from the group and later rejoin, they can be asked to provide the password in order to prove that they're not an infiltrator in disguise.
- Used to great effect by the Black Hands group. During an annual Hackmaster tournament, Newt asks Stevil for the password and, not getting it, peppers his body with crossbow bolts and dumps his body down a well. All to the good, except that Newt accidentally (or "accidentally") forgot to set up the password with Stevil ahead of time, giving him an excuse to take him out of the game. Nitro doesn't buy the excuse and, when the Black Hands are eliminated, orders Newt to his house for "remedial training". And to bring a sack lunch.
- In the Harry Potter Peggy Sue fic Backwards with Purpose, Harry convinces Dumbledore of the time travel by stating what Dumbledore saw in the Mirror of Erised.
- Ed and Sam of qntm's Ed Stories both have time travel passwords, for use when they need to prove their identities to themselves. They make use of these passwords in Be Here Now: 4 of 5.
- Twisted in Futari Wa Pretty Cure Blue Moon. Facing down an illusory copy of Yoko controlled by Millusion, Cure Sunday asks her to answer a question about herself. The twist is that giving the right answer proves her as the fake, because Yoko didn't know it.
- Kyon Big Damn Hero starts with Kyon using his Trust Password so Haruhi will believe him and help Yuki.
- In the Bleach fic Hogyoku Ex Machina, Yamamoto gives Ichigo one of these to prepare for when Rukia will be Ret Goned in the future (i.e. the third film). It also ends up being useful for identification against Aizen's illusions, though Ichigo almost gets killed then anyway since the password is a name and he has trouble remembering it.
- Ichigo convinces Gin that he's from the future by explaining the true power of Gin's bankai, which Gin never revealed to anyone.
- Winter War: How do you convince someone who's been Aizen's prisoner for months that you really are you? You hand him his zanpakutou- which will do the convincing for you.
- In scifigrl47's Toasterverse, Tony has one of the escaped scientists sing the Teapot Song to verify his hand in their escape.
- In the Hard Reset sequel Hard Reset 2: Reset Harder, when Celestia hears that Twilight is a fellow time looper, she tells Twilight a password to unlock secret knowledge in Celestia's brain. When Twilight repeats the password to Celestia later on, she's paralyzed by a powerful spell, and Celestia immediately kills her after revealing that the "secret knowledge" is the rest of the password. It's repeating the ENTIRE combination that convinces Celestia of Twilight's story.
- Bill & Ted's Excellent Adventure (see page quote) has the future selves prove that they are who they are by identifying what number the present selves are thinking of.
- Turned on its head in Bill & Ted's Bogus Journey when the "Evil Robot Usses" correctly guess how many fingers Ted is going to show them; it's entirely possible he only held up the amount they said because they said it.
- Back to the Future
- 2015 Biff Tannen demonstrates to his 1955 self that he and his Sports Almanac really are from the future by predicting an extreme long shot football game result in Back to the Future Part II. Although Old Biff makes a point of not telling his younger self who he really is, he demonstrates knowledge that only Young Biff would have, such as how to start his car.
- From the first film, Doc will not believe Marty is a teenager from the future until Marty proves it. Marty first tries to validate himself by revealing "future" history, which Doc laughs off because he refuses to believe that Hollywood actor Ronald Reagan could possibly ever become President. Doc only believes Marty when the latter mentions things he could know only from Doc himself, particularly Doc's accident and vision that later led to the creation of the time machine itself.
- Similarly, the protagonist of Frequency convinces his father in the past that he really is speaking from the future by detailing a baseball game that hadn't happened yet. Protagonist Sr. buys it when he sees the game happen exactly as described the next day.
- Subverted in Ghost Town. Ricky Gervais' character has started telling the wife of a dead husband (whose ghost he can see) facts that he shouldn't know. So she makes him prove that Ricky's really talking to the husband with a recurring nightmare the husband had that he only told her about. He says drowning, but it turns out that's way off, and the husband was purposely lying to Ricky so that he would stay away from the wife.
- In X-Men, after being fooled by Mystique one too many times, Cyclops demands that Wolverine prove he is who he says he is. Wolverine's response: "You're a dick." It works.
- In Cooperstown, a former baseball player claims to be seeing the ghost of his deceased friend. To figure out if he's lying or not, another character asks him several questions that she thinks Harry won't know, but that the ghost would know. As it turns out, Harry was seeing the ghost of his friend. The questions proved it.
- Star Trek actually gives an example where the Trust Password doesn't work. Spock Prime greets Kirk with "I have been and always shall be your friend". Except Kirk and Spock aren't friends yet in this timeline, so it kind of falls flat. He finally resorts to using a mindmeld instead.
- In Twelve Monkeys, Cole can't figure out if he's actually from the future or if he's just crazy. He figures it out when he tells Kathryn to make a phone call for him to a phone he knows will be monitored in the future. Kathryn leaves the message Cole had been told about in the future, which confirms he's from the future.
- From You Only Live Twice:
M: [buzzing intercom] Miss Moneypenny, give 007 the password we've agreed with Japanese S.I.S.
MoneyPenny: Yes sir.
MoneyPenny: We tried to think of something that you wouldn't forget.
James Bond: Yes?
MoneyPenny: "I, love, you". Repeat it please, to make sure you get it
James Bond: Don't worry, I get it.
Tiger Tanaka: Permit me to introduce myself. I am Tanaka. Please call me Tiger.
James Bond: If you're Tanaka, then how do you feel about me?
Tiger Tanaka: [the code response] I... love you.
James Bond: Well, I'm glad we got that out of the way.
- Big has the most complicated example, with a fast sort of song and dance routine.
- Similar to the Big example, Rob Schneider's character in The Hot Chick (really Rachel McAdams' character's brain in his body thanks to a "Freaky Friday" Flip) recites a handclapping game about how All Men Are Perverts (with Subverted Rhyme Every Occasion) to prove who he really is to his best friend.
- Subverted in 17 Again. Mike tells his best friend Ned several incriminating details that only he could know while being attacked, but Ned justifies how the weird, soaking wet man in his house could get access to all the information he gives.
Mike: It's me, Mike O'Donnell, your best friend! You have an undescended testicle.
Mike: You helped me cheat on a math test, but I got caught.
Ned: Public records!
Ned: Covered by the local news!
- In one of the The Bourne Series films, it's shown that all Treadstone agents have "Safe" and "Under Duress" passwords.
- Groundhog Day takes it to extremes. To prove that he really is repeating the day over and over again, Phil walks Rita through a diner, giving the trust passwords for everyone in the diner, none of whom remember telling him those things. She still doesn't quite believe him until he shows he can also predict what people are about to say.
- Parodied in one or more of The Pink Panther movies. Clouseau asks Dreyfus what his code name is, or what the password is. Dreyfus has to think, then replies in a fury that he doesn't have a codename and there is no password. Clouseau confirms that only the real Dreyfus would know that, causing Dreyfus to become even more angry.
- In Airheads, to determine whether a record executive actually is an exec, not an undercover cop, Chazz asks him "Whose side did you take in the Van Halen / Roth split: Van Halen or Roth?". When he answers "Van Halen" instead of "Roth", he's concluded to be a cop, but Chazz gives him one more chance, asking "Who would win a wrestling match: Lemmy or God?". As he first answers "Lemmy" before answering "God", Rex tells him "Wrong, dickhead! Trick question! Lemmy IS God!", and he's finally driven away.
- In the film Ghost, Patrick Swayze's deceased character is trying to convince his girlfriend that a medium can speak for him. He tries to tell her that he loves her, but this backfires, since he was reluctant to say those words in life. He quickly corrects himself and tells the medium to say "ditto," which is how he would usually respond to his girlfriend telling him that she loves him.
- In The Sixth Sense, Lynn Sear accepts that Cole is telling her the truth — he really can see and talk to dead people — after this exchange:
Cole: [of his grandmother] She wanted me to tell you...
Lynn: Cole, please stop...
Cole: She wanted me to tell you she saw you dance. She said, when you were little, you and her had a fight, right before your dance recital. You thought she didn't come see you dance. She did. She hid in the back so you wouldn't see. She said you were like an angel. She said you came to the place where they buried her. Asked her a question. She said the answer is: "Every day." What did you ask?
Lynn: Do... Do I make her proud?
Live Action TV
- In the Battlestar Galactica (Classic) series, when Starbuck escapes from the Cylons by stealing one of their ships, he proves it is him to the Galactica by "waggling" the ship back and forth (since the ID transmitter he was given was damaged).
- In the Series Four Doctor Who episode "Forest of the Dead," Professor Song whispers the Doctor's name into his ear to prove that she knows him in the future.
- Subverted in later episodes where it is implied that that she might not be someone he ought to trust after all.
- And in the season finale, the metacrisis Doctor whispers something to Rose to make her accept him as the Doctor that's right for her, rather than the original. Presumably, he tells her he loves her, but if you can't cope with that, pretend he told her she would be king of France.
- "She told me to warn you. She said two words: BAD WOLF. Well, what does it mean?"
- And from the series six premiere:
Amy: Trust me.
The Doctor: Okay.
Amy: You have to do this. And you can't ask why.
The Doctor: Are you being threatened? Is someone making you say that?
The Doctor: You're lying.
Amy: I'm not lying.
The Doctor: Swear to me. Swear to me on something that matters.
Amy: Fish fingers and custard.
The Doctor: My life in your hands. Amelia Pond.
- In LOST's "The Constant," 2004-Daniel tells Desmond to tell 1996-Daniel that he knows about Eloise, and give him some important numbers for an experiment. This proves to Daniel that Desmond is traveling in time and has spoken to a future version of Daniel.
- In an episode of Stargate SG-1, the team accidentally travels back in time to 1969. Since this was a Stable Time Loop, General Hammond knew to send them with a sealed note explaining things to his rookie past-self. When he finds it, Lieutenant Hammond believes it and helps them escape imprisonment (and remembers to write the note "again" 30 years later.)
- Also in the same incident, O'Neill convinces Lieutenant Hammond the team know him in the future by telling him he watched the first moon landing two weeks ago from his father's bedside after a heart attack.
- In another episode, the team finds a crystal skull inside a chamber filled with radiation and their attempt to examine it results in Daniel being made invisible and immaterial. The rest of the cast has no idea what happened to him until it is revealed that Daniel's grandfather Nick (who had once seen a similar skull and was living in a mental institution) can see him. Jack is skeptical, but is convinced by this exchange:
Daniel: Repeat after me: He's standing right beside me.
Nick: He is standing right beside me.
Jack: Well, he's lost a few pounds...
Daniel: Jack, don't be an ass.
Nick: Jack, don't be an ass.
- Similarly, Daniel is inclined to believe that a teenager that got on the base is O'Neill because of the exasperated way the kid shouted "Daniel!"
- In the same episode, he convinces a group of seasoned pilots that he really is O'Neill with a well-known phrase. The majors and captains visibly straighten in their seats and lose their smiles.
- Parodied a few times in one episode where the team is dealing with aliens that can pretend to be other people:
Jack: Daniel, are you you?
Daniel: Yes, are you?
Daniel: Never mind.
Jack: Wait, how do I know you're the real Daniel?
Daniel: *exasperated* Because.
Jack: *beat* Yeah, okay...
- In Stargate Continuum, the trope is subverted twice: O'Neill refuses to believe the alternate timeline SG-1 (partly because what Daniel tries to use—his son's accidental death—didn't happen in the alternate timeline), while Landry cuts them off as they are about to do this by telling them he believes them (having previously seen the tapes of their interrogations).
- In an alternate future of Star Trek: Enterprise, the now elderly Captain Jonathan Archer is being cared for by his... caretaker... T'Pol. However, an injury prevents him from remembering all that's happened since he was last on the Enterprise. T'Pol says to him that she fully understands that he might consider all this to be an illusion or an elaborate deception. To alleviate his fears she tells him the story of an old girlfriend he wanted to marry back on Earth. The stunned Archer wants to know just what kind of 'relationship' he and T'Pol have that he'd ever tell her the story. She'll only say their relationship has "evolved".
- In the Star Trek: The Original Series episode "Whom Gods Destroy", a shapeshifting madman assumes Kirk's appearance and tells Scotty to beam him back aboard. Much to his chagrin, Scotty challenges him with "Queen to queen's level three, sir" — which only Kirk and Spock knew the appropriate countersign for.
- Time Travel was the whole point of Seven Days, and the team set up a code phrase for the protagonist to use so that they'd always know when he had shown up from the future, and they weren't dealing with a hoax. The password used is Frank contacting Backstep and identifying himself as 'Conundrum'.
- This password is only useful for the first episode, when the team has no idea who he is. After that, he's really just using 'Conundrum' as a codename and to get put through to the right people, because they a) all know Frank, and b) notice when their giant time-traveling sphere vanishes from the hanger.
- In one episode of Lois and Clark, a time-traveller is able to enlist Clark's aid by whispering "I know you're Superman, and I need your help", to him.
- In the Charmed episode "That 70's Episode", Prue and Piper convince their past selves to trust them by opening a trick drawer in a cabinet.
- In the Supernatural episode "The End", Dean is sent to the future and proves his identity to Future!Dean by telling a story that only he would know:
- In an episode of Buck Rogers in the 25th Century, Buck hijacks a Draconian fighter and must prove to another pilot that he's from Earth. The pilot is someone Buck had previously been stranded in the desert with and had shared stories of the past. In this case, Buck proved who he was by giving a description of OJ Simpson. Since Buck had left Earth in 1987, he had no idea that OJ turned out to be a double murderer and since the show was filmed prior to this, neither did the writers.
- And in Buffy the Vampire Slayer, this classic exchange. Makes even more sense when you realize the above X-Men "You're a dick" line was also penned by Joss Whedon.
Giles: Cordelia, it's me! It's me!
Cordelia: How do we know it's really you and not zombie Giles?
Giles: Cordelia, do stop being tiresome.
Cordelia: It's him.
- In another episode, Xander does the Snoopy Dance to prove he's Xander for Willow.
- He also tells a story about one of his early birthdays where he didn't get a firetruck and was sad, but then the neighbours house caught fire so real trucks came and he always thought Willow might have set the fire "and if you did you can tell me". Played with in that he's doing all this to prove that he's not his Evil Twin who the rest of the Scoobies have gone off to fight. A) His twin would know all this stuff anyway, given that Xander had been duplicated and B) Willow wasn't even aware there was a twin so she had no reason to doubt he was Xander. She just let him talk because he didn't give her the option not to.
- In one episode of Bones, a boy is kidnapped. Booth asks the dad what password he can use so the boy will trust him during the rescue. (It's Paladin, and the rescue scene is awesome and heartwarming.)
- And the appropriateness of this trust password for Agent Booth, a Roman Catholic FBI agent, is lampshaded by his boss:
"Paladin." Defender of the Faith.
- An episode of Star Trek: The Next Generation had Worf visited by a Klingon who first claims to be an advisor, and then later admits that he is Worf's now-grown son Alexander, traveling back in time to urge current!Alexander into becoming a warrior. To prove his identity, Worf asks future!Alexander to tell him what Alexander's mother said before she died.
Alexander: I was three years old. She was dying when we found her. She barely managed to whisper my name. And then she took my hand and placed it in yours. And she died. And then you howled in rage, and said "Look upon her. Look upon death, and always remember." And I always have.
- In The West Wing, the word "Sagittarius" is used to denote that person knows about Jed Bartlet's MS.
- In the series 2 premiere of Misfits, the gang have to deal with a shapeshifter. In order to make sure it's really them, they greet each other with "Monkeyslut!"
- An unintentional variation occurs in Farscape. John is having a normal conversation with Aeryn until he mentions her baby and she doesn't know what he's talking about. This gives away that she is a bioloid duplicate and the real Aeryn has been kidnapped.
- In the Smallville episode "Transference", Clark Kent and Lionel Luthor swap bodies. In Lionel's body, Clark convinces Ma Kent it's really him by telling the story of how he discovered Super Speed at age 6.
- Subverted in "Apocalypse". Clark tries to prove he's a friend to Chloe by revealing some of their past experiences, but since he's in an alternate universe where they have never met, it fails.
- In one episode of Without a Trace, the episode's missing person is a preteen girl who's been kidnapped as part of a pedophile ring. At the episode's outset, while talking about her quirks, the girl's uncle tells Malone that his nickname for the child is "Chicken Little." Much later, this comes in handy when Malone, having determined where the girl's abductor is, poses as a pedophile customer and gets to talk to the girl, but he can't say anything too suspicious to her since they're both still in the antagonist's line of sight. What does he wind up saying to gain her trust?
Malone: Chicken Little says hi. (the girl immediately looks hopeful)
- On 30 Rock, Nathan Lane shows up claiming to be Jack's brother, and Liz is suspicious because Jack has never mentioned a brother and this man pronounces their last name "Dona-hee" as opposed to "Dona-gee." Then he comments that she would be prettier if she didn't scowl, and she knows he's related to Jack.
- In Brad Paisley's "Letter to Me", he says that if he could send a letter back to himself when he was still in high school, he'd prove to his past self it was really him by telling him to look under the bed for the Skoal (chewing tobacco) can and Playboy no one else knew he had.
- In Escape from Monkey Island, Guybrush meets his future self from Twenty Minutes into the Future (Or you meet your past self from Twenty Minutes in the Past). Either one asks "If you're really me, then what number am I thinking of?".
- In Prince of Persia: The Sands of Time, the Prince proves his story to the princess (events that never happened due to the events in the game) by telling her a word invented by her mother, Kakolukia.
- In Betrayal at Krondor, James sends Gorath, a dark elf, to warn Prince Arutha of an attack, and tells him to use the phrase "There's a party at Mother's" to convince Arutha that the message is indeed from James, as it's a phrase they used years earlier in their adventures together and Something Only They Would Say.
- A non-verbal example in Geist. Since Raimi has the ability to Body Surf, he has to prove his identity to Bryson by reproducing their Secret Handshake.
- Near the ending of Planescape: Torment, the only way you can knock some sanity into the Paranoid Incarnation is to talk to him in the obscure language of Uyo, one of the things he used to lock away some journals of his. After all, if he killed everyone who ever knew the language, how could the new guy in the crystal be anything other than a more lucid aspect of himself? The change from "paranoid psychopath" to "scared puppy" is heartbreaking.
- Early on in Final Fantasy VIII the Player Party is sent on a mission to aide an anti government group called "The Forest Owls" and are given a Password to confirm their identity. Upon reaching the rendezvous point and saying the password to the group's representative (regardless of whether or not you gave the correct response), he takes you to meet the other members.
- In the Portal 2 Perpetual Testing Initiative DLC, Cave Johnson realizes that it's very difficult to discern who is the original Cave, and who's from an Alternate Universe. He therefore announces that his trust password is "chariots", and only messages where he says that word comes from the real him. Of course, he then discovers that there's an alternate him who just likes to say "chariots" randomly, so he changes the password to him saying it twice in a row; "chariots, chariots". You will never hear so many references to ancient Greek vehicles.
- In Full Throttle, at one point Ben is about to be (painfully) killed by a gang led by Maureen, who believes that Ben killed her father Malcolm. Of course, Ben's innocent, and to prove it, you have to call Maureen by her childhood nickname, "Diapered Dynamo," which Malcolm told you shortly before he died.
- Fire Emblem Awakening has several for most child characters. Generally they prove their identity with a future version of a unique item, generally a wedding ring but in one case Falchion. Amusingly, one character points out the problem with the item passwords; it is possible that the person presenting it murdered the time-traveller and stole their mom's wedding ring
- There's a way to use this in Millennia Altered Destinies to get the most powerful weapon in the game. Occasionally, when you perform a temporal jump, you will end up in a green mist with a mirror of your ship in front of you. You can contact your other self, and he will tell you a number. The next time you're in the same mist, you can tell your other self the same number. He will then teleport a set of plans onto your ship that can be given to a sufficiently-advanced race to build.
- In the Mass Effect series, quarians are issued one of these to give when returning to the migrant fleet. They are also issued another phrase that indicates they are returning under duress and their ship should be fired upon before it gets too close.
- In the second game, during the first meeting with Tali, Shepard can mention the geth data that they gave her during the first game, wondering if it helped her complete her Pilgrimage. As Tali was the only other person who knew that Shepard had disobeyed orders by giving her a copy of classified intel, this proves that Shepard had indeed come Back from the Dead.
- In the Citadel DLC for the third game, Shepard can prove to Traynor that it was their Evil Clone that just fired her and threw her off the Normandy, by describing the toothbrush that Traynor mentioned as wanting to buy during their first conversation. Alternatively, a Female Shepard in a romance with her can do this via a "Shut Up" Kiss.
- Bob and George hangs a lampshade on Bill and Ted when time travel becomes a regular occurrence in the comic; of course, since Mega Man and Bass are legendarily stupid, Hilarity Ensues.
- In Home On The Strange, Tanner sets one of these up with Izzy, to the latter's bemusement.
- During their adventures in the Punyverse in Sluggy Freelance, Bun-Bun cons Lord Grater into believing he works for his boss, Zorgon Gola. Lord Grater responds "if you know everything about me, what am I thinking about right now?" Bun-bun responds that the infomation is classified, which instantly appeases him.
- Inverted another time in "Oceans Unmoving". (Big spoiler alert.) Bun-bun can't appear himself to his past self as himself, because he knows that if he were to see someone claiming to be himself from the future, he'd figure it was a trap and kill them, expecting that if it were really him he would have expected that.
- Panthera uses this. When Onca tries to tell Tigris that Ovid isn't the evil corporation they've been told it is, Tigris thinks Onca betrayed Panthera and attacks her. However, when Pardus shows up, she tells him that Leo told her Pardus dyes his fur. Pardus knows Leo wouldn't tell her unless he had to to validate a message to the others.
- Double-subverted in The Inexplicable Adventures of Bob!. In the dragon Hibachi's first appearance, he claims to be a friend of Mrs. Primrose who'd been sent to pick up the box she had left with Bob. Bob is understandably suspicious, and demands the seemingly-useless Trust Password of her favorite color. Turns out it's such an obscure color (a specific discontinued shade of paint), and has such an involved story attached to it that it really would be impossible for anyone who didn't know her to guess.
- In Homestuck, Jade implements a system of these, though it's less about trust and more about trying to keep her conversations relatively linear since the Trolls tend to skip around a lot regarding time shenanigans.
- In Red vs.Blue: Reconstruction Agent Washington verifies that he is from Command to the Reds by knowing Sarges ultimate secret Password. It's "password".
- Something Awful goons answer the question "Do you have stairs in your house?" with the countersign "I am protected."
- Aladdin, with "Do you trust me?", which works both when he's a street rat and a prince, which also starts to show that they're the same person.
- In the Justice League episode where Flash and Lex Luthor switch minds, Flash proves he's really himself again by starting to reveal Green Lantern's old nickname.
- In Ben 10: Alien Force, Past!Gwen demands Future!Gwen say something only they/she would know. Future!Gwen whispers something to her, prompting a disgusted reaction. We never find out what it was. Dwayne MacDuffie refused to comment, saying it was "personal."
- In the My Little Pony: Friendship Is Magic two-parter A Canterlot Wedding, Twilight and Cadance have a rhyme from way back when. When the false Cadance doesn't know it, Twilight smells a rat. When, due to False!Cadence's manipulations, she nearly blasts the head off the real one, Real!Cadence does this to prove it's really her this time.
- In the Family Guy episode "Prick Up Your Ears", Stewie tries to catch the Tooth Fairy. He sets up a trap, which catches Brian.
Brian: Stewie, what the hell? Get me down from here.
Stewie: No, way, man! How do I know you're not the Tooth Fairy in disguise?
Brian: Your middle name is Gilligan.
Stewie: Not good enough!
Brian: You think my girlfriend's a moron.
Stewie: So does everyone!
Brian: You have a picture of Chris Noth in your wallet.
- On South Park Cartman wants to be a Human Popsicle so he won't have to wait for the Nintendo Wii to come out. He ends up in a Bad Future (there is a war going on, but Cartman is mad he can't play the Wii) and calls his past self to fix things. He tries to convince his past self who he is by explaining that he drank Ovaltine and put Clyde Frog in the closet before going to Butters and trying to freeze himself. His past self merely thinks someone was spying on him.
- Batman: The Brave and the Bold: In "The Criss-Cross Conspiracy!",Batman is trapped in Batwoman's body. Nightwing asks him to prove it by saying something only Batman would know. Batman responds "Your favorite color is blue, you used to sleep with a nightlight and you're deathly afraid of monkeys". Nightwing goes "It's him".
- Likewise, when Batman is Astral projecting himself due to being buried alive, he possesses Speedy and starts telling Green Arrow about what's going on, Arrow initially thinks it's Speedy doing an impression, then Bats grabs him by the hem of his shirt and threatens him to his face; "It really is you, isn't it?".
- According to tradition, Joan of Arc whispered some secret to the Dauphin to convince him that she had really been divinely inspired to help him claim the throne. According to additional tradition, what she whispers is that God wants him to be king of France, though some prefer to believe that she described an embarrassing birthmark on his backside. Another version holds that she repeated a prayer he'd said in private, which only God could have revealed to Joan.
- Magician Harry Houdini had spent much of his later career debunking mediums and others who claimed to speak "from beyond the grave". He arranged a number of code phrases (one being a song called "Rosabelle") as identifiers for his wife Bess if such communication was possible. He died in 1926, well before Bess. No one was ever able to deliver a message she was satisfied was genuine.
- Used well in Carrie Vaughn's Kitty Norville series, where a medium contacts Houdini, and is told "Everyone who knew my codes is dead. No one will believe you. But thanks for trying".
- Reportedly most mediums who tried to contact Houdini's mother for him made the mistake of having her address him as "Harry" - which his real mother never would, because it was just a stage name. She would only ever have called him Erik.
- That password you have to tell the computer to make sure it's really you.
- Careful parents often arrange these with their children, so that the children can verify that anyone who claims to be sent by mom and dad actually was (as in the Bones example above). Some PSAs (or similar productions) recommend this if the family ever gets separated.
- Surprisingly more common than most people think. It's extremely easy to set up, and it never hurts to be Crazy-Prepared or Genre Savvy.
- There was a website many years ago (it's long since dead) that was touted as a "time-traveler's support network." Meeting places and times in various cities were designated where a "volunteer" from the project would wait for someone to say a one-time use passphrase and render aid. The database of locations, times, and phrases was said to exist in a sealed document held by a law firm which would be turned over to any time-travel project in the future.
- It may be long since dead now, but if you're going back in time that shouldn't really be a problem.
- As in the Bourne example above, secret agents normally have security checks they can insert into a message to verify that it's real. During World War II, one British agent captured by the Germans deliberately gave his captors the wrong security check. He expected his bosses to realize that the messages coming from his radio were false. His bosses didn't pay attention. Das Englandspiel (also called Operation North Pole) resulted in the capture and execution of approximately fifty Allied agents, and didn't end until the Germans themselves called it off in a clear-text message to London..
- This is also true of military operations. Generally speaking, rather than attempt to describe a situation, ground forces will have code phrases during extraction to describe the situation to the (usually a helicopter) extraction unit. The most common three types are Situation Under Control, In Contact, and Do Not Attempt Extraction. They're coded so that, if the ground forces have been captured, they can wave off the extraction unit without pointing out that they're doing so.
- Shibboleths are words whose pronunciation is so unique that only native or extremely fluent speakers can pronounce them correctly.