So the heroes are crawling through a dungeon, or infiltrating the Evil Overlord's Supervillain Lair, or popping down to the shops for some milk or what have you, when they come upon a pair of doors, or a fork in the road, with each path guarded by a heavily-armed soldier (or animated statue, or whatever). They're somehow informed that one door leads to a truly inescapable Death Trap, while the other leads the way they're going, and they have to ask the guards which door is which.
The trick is, one of the guards always tells the truth, and - wait for it - the other guard always lies, and the heroes are allowed to ask only one question.
Sound agonisingly familiar? This is the popular Knights and Knaves logic puzzle (the Trope Namer is a particular version by mathematician Raymond Smullyan, but the puzzle considerably predates him), and invariably the scenario used every time in the media, to the point that it's a Dead Horse Trope. If you're lucky, the puzzle will spring for a bit of originality and involve a third guard who alternates between telling the truth and lying (or worse, a "normal," who can do either or neither at will). Smullyan himself invented dozens of variations and would probably be disappointed that it's just the one that ever gets cited.
For the record, the most common solution to the above scenario is to ask one of the guards, "If I asked you if the door you're guarding leads to where I want to go, would you say 'yes'?" If he says yes, then you go through his door, while you go through the other door if he says no. This is because his answer to this question doesn't depend on which guard he is. Say he says yes to the question. If he's telling the truth, then he would say that the door leads to where you're going, and thus, the door will lead to where you're going. If he's lying, then he'll have to lie about whether he'd say Yes to the question (which, in this case, he would not say yes if asked if the door led to where you're going, and would in fact say no) and, thus, is forced to give the correct answer to where the door goes. Of course, this requires that both guards know where you are going, and that neither of them considers "Your doom" a place.
The second most common solution is to ask either of the guards "If I had asked the other guard which door was the correct door, which door would he have pointed to?" They will both give the same answer (indicating the wrong door). If you're talking to the guard that tells the truth, he will (truthfully) indicate the door that the other guard would have steered you towards — which would be the wrong door, as the other guard always lies. But if you're talking to the guard that always lies, then he would still point to the wrong door, as while the other guard (the truth-teller) would have indicated the correct door, the guard you're speaking to is lying to you about what he would have said! So either way, the answer to your question will be the wrong door — and so, either way, you simply use the other door.
Note that if a character in these puzzles is said to always lie, then it is (probably) Not Hyperbole, unlike in real life. Real life "liars" are intending to make people trust them, and thus are perfectly willing to at least occasionally tell the truth. One of these guys, on the other hand, will be Lawful Stupid, Chaotic Stupid with regard to the habit of lying, and thus can be caught out as depicted in the picture, or by less violent means. However, note that one of the keys to this puzzle being a puzzle is that you have to get a piece of information out of these two guards, rather than just determine which one is lying, which is what prevents you from simply asking them what 2+2 is. In addition, it is impossible to know who the liar is by asking him, as he would always lie about his being a liar. Some works can forget this, and make the hero look like something of an idiot for going for needless complexity instead of Cutting the Knot.
It should also be noted that no author (except those of logic puzzle books) ever includes amore complicated or different version of the puzzle. Smullyan created numerous permutations of his own puzzle, including one with islanders who answer only "Da" or "Bal" instead of "Yes" and "No," and the point is to figure out puzzles without necessarily knowing which means what in English. Another is set in Transylvania, where people can be either sane or insane (insane people believe untruths) and either a human or a vampire (humans say what they believe is true, vampires say what they believe is false). Most often writers can be excused for not including these more difficult ones, as they would be very difficult for the audience to understand. Not that we would mind.
Heroes who have neither the patience nor aptitude for logic puzzles generally just skip straight to the violence when confronted with this one. In videogames, it can also be brute-forced by Save Scumming.
Of course, the puzzle was "meant" for people for whom a pair of armed guards are a formidable obstacle, rather than for the standard "hero" types. If the guards aren't monsters, obviously.
The real question is, was the guy who explained the rules telling the truth or lying?
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Subverted in Yu-Gi-Oh!. During the Duelist Kingdom arc, the Paradox Brothers confront Yugi and Jounouchi/Joey with this puzzle. Yugi correctly guesses that the brothers' description of the puzzle is, in fact, part of it, and that both the brothers are lying about the whole puzzle (since they both say that one always lies and one always tells the truth, which is impossible, since someone who always lies would be unable to give a honest description of the puzzle, and thus could never agree with someone who always told the truth), and outwits them his own way.
They're cheaters anyway. Whenever a person asks his question and chooses a door, they always claim the opposite door is the right one. Yugi tricks them by making them think he's choosing one door, waiting for their answer, and revealing he chose the other one. Both doors lead to the exact same place anyway.
A variation is made in Ghost in the Shell: Stand Alone Complex where the endlessly curious Tachikoma's steals the hardware for a snipping assistance device by using the statement as a logic bomb.
Done in the first story arc of Grant Morrisson's Doom Patrol: the twin priests of Orqwith must be asked a question in order to destroy their invasive reality. One says, "I am an honest man and I do not know why there is something instead of nothing"; the other says, "I am a liar and I do not know why there is something instead of nothing." Rebis correctly reasons that the honest one would not be able to call himself a liar, so the one who does must be the liar - but this means the other part of the liar's conjunctive statement must be false in order to make the statement false overall. So the liar is the one who knows the answer to the final question.
Bonus points for borrowing that literally from the abovementioned professor Smullyan.
Sally Acorn of the Sonic the Hedgehog comic solved one of these in the "In Your Face" special.
In one 1990s Superman comic, Mr Mxyzptlk, who had just discovered the exciting third-dimension concept of lying, did the three person version: three Mxys with switches in front of them. Two switches will electrocute Lex Luthor II, and two Mxys will lie about which one's safe. Superman correctly deduces Mxy #2 has the safe switch ... and Mxy #3 claims he's wrong and moves to pull his switch! Superspeed takes care of this flagrent cheating.
A Mathnet comic (from Square One TV) included in a tie-in magazine issue of 3-2-1 Contact involved this puzzle. Kate Monday and George Frankly had to find out which of two identical twins were stealing birds from pet stores. One revealed that he always told the truth and his brother always lied - leaving the two detectives to figure out who was the thief. This particular Knights and Knaves puzzle was a variation on the traditional format; no limit on questions allowed was specified. The solution given was to ask the brothers a trick question like "Are you a parrot?" It was reasoned that the brother who always lied would say "yes" and the always truthful brother would say "no".
A Future of Friendship, A History of Hate uses a variant in Episode 2 for the first challenge Twilight has to pass to save her friends from Ruinate. A two-headed sphinx does the usual "one head tells truth, the other lies/one path leads to safety, the other to doom" bit, but the variant comes in when she realizes the answer: she realizes that since both heads were in agreement on the rules, which the liar wouldn't be, they must both be liars, meaning both paths are dangerous, and the sphinx is actually hiding the safe third path.
Films — Live-Action
Arcade — though this is a bit of an aversion, since the guards tell her flat-out which guard is the truth-teller and which is the liar. And then for some reason the heroine asks the liar which way to go.
In Werner Herzog's Every Man For Himself And God Against All, Kaspar Hauser is asked this question by a doctor trying to test his intelligence. The doctor will accept only a complex answer, but Kaspar responds simply (and correctly, since the doctor did not include the proper constraints), "I would ask him if he is a tree-frog."
Shows up in Labyrinth. It's played with, though, as Sarah falls down a trap door behind the door at the precise moment she announces herself triumphant. Still, she did get the right answer, and she did end up getting to the end.
The blue guard told Sarah the conditions: "One of us always lies, and one of us always tells the truth." If the conditions were valid, then that particular speaker was the truth-teller. If he was lying, then all bets were off.
All bets were off even before that. The Blue Guard tells Sarah about the conditions right after he agrees with the Red Guard that Sarah can only ask one of them. If one of them is always honest and the other always a liar, neither of them should agree on anything at any time.
They also try to mislead her with a little joke:
Blue Guard: He always lies. Red Guard: I do not, I tell the truth! Blue Guard: Oh, what a lie!
One interpretation of the scene is that the guards are just messing with Sarah and that NONE of them actually knows which door leads to the center of the labyrinth.
In Open Graves, the last step in the cursed board game is guessing which of two snakes' mouths to place your playing piece into, assisted in your choice by a Knights and Knaves question.
A variant occurs in one of the Lone Wolf gamebooks. A performer brings out two children, masked so as to conceal their genders. One states "I'm a boy" and the other "I'm a girl." The performer confirms that they are indeed a boy and a girl, but at least one of them is lying, leaving Lone Wolf to determine the gender of each without asking any further questions. Of course, given the above information, if one of them is lying, the other must be as well, making this one as straightforward to solve as the classic version.
The short story How Kazir Won His Wife by Raymond Smullyan involves various more complicated variations on the puzzle, while the framing story is set on an island where the normal version has occurred.
Spoofed in the Discworld novel Lords and Ladies. To pass the time on their trip to Lancre, Ponder Stibbons mentions this puzzle to Ridcully and Casanunda. Much to Ponder's annoyance, Casanunda insists that the "logical" solution is to wrestle a weapon from one of the guards and force him at swordpoint to show them which door leads to safety. And inform him that he is going in first, just in case he tries any funny business.
Crops up in The Book of Lost Things, due to the fact that the main setting is deliberately based on the tropes of fairy tales.However, its played with bridges rather than doors: two bridges over a harpie-infested ravine and each one guarded by heavily-armed trolls. Only one of the trolls knows which bridge is safe enough to cross, etc, etc. The bookworm protagonist answers the riddle correctly- making it one of the few circumstances in which he fares better than his friend the Woodsman prior to taking a level in badass.
Played straight in Cecilia Dart-Thornton's Bitter Bynde trilogy, as a challenge to someone attempting to escape the realm of the faerie. The protagonist must determine which door leads her to freedom with a single question posed to the titular knight and knave (who, in this case, are trapped humans).
In a brainteaser by puzzle writer Dr. Crypton, the protagonist is visiting a one acre desert island, seeking his way to the island's only tourist attraction, a tower. He comes to a crossroads, where four roads split off, and there are three natives there. The four possible tribes of natives: always tell the truth, always lie, can answer with truth or lies, or wait for someone else to say something and then say the same thing. And he can ask them only two questions. The answer is to ignore them completely, as a -tall tower on a one acre desert island is impossible to miss.
In Martin Gardner's Hexaflexagons and Other Mathematical Diversions, he gives the version where you are at a fork in the road with one native (who either lies or tells the truth), and want to find out which road leads to the village. As well as the traditional answer, he suggests you ask the yes-or-no question "Did you know they are serving free beer in the village?" and then just follow the road that the native sprints down.
Many puzzle books tend to include variations on this problem. For example, The Lady or the Tiger?: and Other Logic Puzzles starts off with signs, later switching to a sane/insane people, then inverts it to people who can only ask questions that may be answered yes or no (specific to the person asking said question).
In the backstory The Gone-Away World by Nick Harkaway, Mr. Soames is confronted by three anthropophagous witches who offer to give him directions using the standard Knights and Knaves setup. Being a logician, he takes them up on the offer. The whole thing is a trap, which is why Mr. Soames is dead in the main story.
The Doctor Who serial Pyramids of Mars featured this as one of several puzzles the Doctor had to solve to enter the titular structure. This incident is an example of solution #2, asking the one guard about what the other guard would have said. Why an ancient Martian pyramid imprisoning a Sufficiently Advanced Alien was protected only by logic puzzles is unknown. The Doctor, being the clever bastard that he is, figures it out in about 15 seconds. According to the DVD production notes subtitles, Phillip Hinchcliffe got it from Franz Kafka's The Castle, although this cannot be confirmed.
Straight example in the math-and-logic Edutainment ShowSquare One TV, with the three-person variant. The alternating character, when asked who he was, said he was the knave, which neither the knight nor the knave would say. Then the knave claimed to be the alternator, which the hero had already identified, leaving the last person to be the knight. Of course, this is a little contrived, as both the Knave and the alternating character could claim to be the Knight, in which case you'd be stuffed, since all three would be claiming the same thing.
Subverted in the Mini SeriesThe 10th Kingdom. Two doors to safety or death are guarded by a talking frog who offers one question, but claims to always lie (which would make it unsolvable as a logic problem since the rules themselves are in doubt). By now the father of the protagonist Virginia has had it with this kind of puzzle.
Tony: All right, all right. Wait, wait! I have a question! What is the point in having a door that has a horrible death behind it? Huh? (picks up frog) Frog: Get your hands off me! Tony: What does that achieve? Frog: What are you doing? Tony: I mean, what is the purpose of your life? Just to be a pain? Frog: Don't touch me there, only my girlfriend touches me there! (Tony throws the frog through one of the doors) WHOA! (Tony slams the door, there's a large explosion and fireball) Wolf: I guess it's the other one.
Discussed in the Canadian kids' show Radio Active, where the students are assigned the problem in class but the proper answer is never figured out.
Is analysed as one of the puzzles on Dara O'Briain's show School Of Hard Sums with the catch that you can only ask one question. The answer given in the show is "Will the other person claim their door is the correct one?" which always results in a lie.
Appears in The Legend Of William Tell when Kalem is trying to teach Will to think about things. His companions, including his Smart Guy, have already gone through one of the doors, but they slam in his face and he has to logic his way through.
In the series NUMB3RS, the FBI catches a pair of criminals who stole a truck full of aid money. One says that the truck is gone while the other says the truck is still there. Charlie, a mathematican, is able to deduce that the scenario is identical to this one and uses the correct answer, ask what the other person is going to say. When the answer from both suspects is the same, that the truck is gone, they know it must still be there.
An early (as in, from classic Greek times) version of this is the so-called "Epimenides Liar Paradox", in which Epimenides (a Cretan) claims that "all Cretans are liars". Discussed by Raymond Smullyan in What is the Name of This Book?, in which he points out that it in fact isn't a paradox, but is completely consistent with the assumptions that (1) Epimenides is lying and (2) at least one Cretan tells the truth.
This one also appears in Disc World, where the character who states that all Klatchians are liars, attempting to show his clever solution, promptly gets beaten up by the local Klatchians.
Perplex City has a version with seven speakers, at least three of whom are knights and three of whom are knaves.
Subverted in an Exalted adventure. The party discovers this puzzle in an ancient refuge for Solars, incredibly powerful near-demigods who were deposed centuries ago. The entire puzzle is, in fact, a lie. Both of the doors have very powerful traps on them. As the book points out, the actual logic puzzle here is not the obvious one. After all, why would a group of paranoid Solars need to solve a riddle to get past their own traps? Likewise, anybody who didn't know which door to go through was Not To Be Trusted, and thus should be directed to the Doors of Doom. Presumably anybody who was allowed in had been told to use the secret door on a different wall.
A similar situation exists in Warhammer 40,000 with Kairos, a Lord of Change, now known as the Fateweaver. He knows everything, but when asked a question, one head gives the correct answer, while his other head give an equally believable lie. And, what with him being a demon of Tzeentch, nowhere is it actually stated that the correct answer is given by the same head each time...
Module I3 Pharaoh. Inside the tomb of Amun-re the PCs can encounter an androshpinx who offers to play a Riddle Me This game with them. If they can answer one of his riddles he will answer a question from them about the tomb. Riddles he can ask include one of these puzzles. People who live on the west side of Bindon always tell the truth, people who live on the east side of Bindon always lie. However, people who live on one side of town can sometimes be found wandering around the other side. If you're in Bindon, how can you find out which side of town you're currently on by asking someone? Answer: ask a passerby "Do you live here?" If you're on the west side the answer will always be "yes", on the east side the answer will always be "no". Then just hope the person you ask isn't a visitor from out of town...
The town of Zozo in Final Fantasy VI is part of a Knights and Knaves puzzle where everyone in town is a Knave, except for one person who doesn't even contribute to the puzzle. It is implied that said Knight is not even from said Krave Land to begin with. So every native of the town is indeed a Liar. Actually, there are other people there that don't lie, but none of them are native to it either. Unfortunate Implications set in when you consider that the population of Zozo is made up entirely of the lower classes of a nearby town. Presumably they were banished for being pathological liars.
Featured/spoofed in the browser-based MMORPGKingdom of Loathing. During the final quest, you have to guess the password to a door from clues garnered from four guards. One always tells the truth, one always lies, one alternates between the two, and the fourth one... craves human flesh (and never says anything but "Graaaaagh").
People have worked out the game uses two versions of this scenario. One requires the usual logic to work out, and one can be solved instantly when you know one fact: One of the guards says "You're full of it" at one point. Regardless of the numbers, he's the truth-teller.
The video game series Ultima features a two-headed horse called the Pushmi-Pullyu, whose heads are a Knight and a Knave. The puzzle is substantially simplified to fit the interface — however you put the question to it, he answers by telling you what his other head would say. And since his explanation of his nature is the same whichever head is speaking, there is something of a flaw in the setup. Not that it really matters anyway, as he tells you only which of two routes is less dangerous, but by the time the player reaches him, neither route is particularly dangerous, and the Money Spider enemies actually make the "wrong" answer more attractive.
Played quite straight with three different agents (liar, truth-sayer and alternator) in Pathologic. Except that you can cheat and use a disguise to figure out which is the liar.
A variation of this problem appears as a puzzle in Escape from Monkey Island, where Guybrush needs to find hidden treasure with the help of two parrots named Huggyn and Kyssin, who are enchanted by voodoo magic to always tell the truth and lie, respectively. The catch with this variation is that you're asking for directions where there are at least three choices at each intersection. Also, the parrots are identical and fly up and off the screen, then come back after answering a question, so you can no longer tell which one tells the truth. The trick is to intoxicate one of the parrots with caffeine or alcohol, which produces an obvious change in the bird's appearance — don't worry, it wears off as soon as you finish the puzzle.
This puzzle appears in Zork Zero. The catch here is that which one lies and which one tells the truth is randomized each time you enter the room, and while in the room, you're not allowed to save.
A valley near Esthar in Final Fantasy VIII contains a multitude of talking rocks that put Squall's wits to task with this riddle... in theory. In practice they're pretty much all full of it, and it's easiest to solve the puzzle simply by wandering around pressing the X button until you hit the right spot.
Played with in Shadow Hearts: Covenant: Lucia's bonus dungeon is a multi-junctioned forest where you are told (by a white flower) that white flowers will always try to help you while the black flowers will always try to mislead you. This is true right up until the last junction, when the black flower gets sick of you and tells you the truth just to get you out of the forest. Meanwhile, the white flowers are actually evil and take this moment - now that they have your trust - to try and lead you straight into a trap.
The real kicker is that the last white flower was actually telling the truth; its Exact Words were that the right path would allow you to "proceed into the forest", not to escape it. Additionally, the white flower that explains the rules at the start never said that the black flowers lied, only that they would try to get you to leave the forest which is something you actually want to accomplish at the last junction. Close reading is essential here.
Neopets does this too in the Tale of Woe (an old Plot). There was this Mutant Hissi, which you had to question. (For those who don't know, a Mutant Hissi has two heads.) The solution is to stab one of the heads, and then ask: "Did it hurt?" If the head answers no, it lies.
The Elder Scrolls Adventures: Redguard, Clavicus Vile poses this riddle to the protagonist Cyrus. He's Genre Savvy enough to ask if Cyrus had a classical education first, knowing it wouldn't be much of a riddle if he'd heard it before.
The shareware game MasterSpy by Albert Ball partly hinges on this; there are three information sources (letters, telephones and radios) and one of the keys to success is to figure out which one of the three is telling the truth. Of course, even the two liars give useful information by virtue of the fact that you know it to be false.
Sonic the Hedgehog (2006) got really stupid with this one. The Soleanna police force, intent on giving Sonic the runaround, have informed him that to progress beyond this point of the game he must ascertain which of them is the man authorized to open the door preventing him from doing so. Not only that, at least one of the five is going to lie to him. The answer ends up being that the whole thing is meaningless. The captain is both the liar, and the guy who told you the terms of their little game in the first place, who just so happens to be standing right next to the door you need open. He literally just has to raise his voice to get you through the door; the game was just for his own sick amusement. While the princess is being held prisoner, no less.
Ib has a Room of Liars early on with six inscriptions below six different portraits telling you which tile to pull in the next room over or which portraits can be trusted. As expected, all of them are lying with one sole exception. Once you figure out the trick and solve the puzzle, your next visit to the room greets you with the sight of the truth-teller's portrait splattered with blood and the other portraits with blood on their hands...
In the freeware Visual NovelRE: Alistair, Travis presents Merui with a version of the puzzle (involving a Knight as the one who always tells the truth and a Demon as the one who always lies) as a challenge: if she can answer it, he'll help fix the computer issue she's having. Merui can't figure it out until Shiro provides her with the answer (by which time the network is back up anyway). Travis actually presents the puzzle incorrectly by making the goal simply to determine who is the Knight and who is the Demon, enabling Merui to (eventually) come up with the third option of asking one of them if two plus two is four or something of that nature.
Dark Seed 2 introduces an interesting variation. Two guards, Ik and Uk, guard a door. The player has to tell which one is which, and if it is day or night. However, there is no sun in their world, and the role of Knight and Knave changes depending if it is day or night. Unfortunately, since the game won't let the player figure it out on his own, he must ask someone else which is the Knight and Knave during day and night.
Professor McLogic Saves the Day is built entirely on this trope: not only do you need to discern truth-tellers from liars and alternators, you also need to deal with animals whose truth-telling/lying ways are reversed by gender and rabidness, philosophers who speak only in "if-then" statements, politicans who love telling you what they think others would say instead of their actual party affiliation/honesty, creatures who tell the truth only at certain phases of the day...and that's just a small sample of the numerous variations this game manages to bring to the table.
A sidequest in Borderlands 2, "BFFs", has four robbers in a Truxican Standoff over which of them stole the money from a heist they recently pulled off. One of them is telling the truth, and the other three are lying. You can just shoot any of them in the head to complete the quest, but if you properly figure out who took the money, you'll get a better reward. It's Lee, the only one who didn't specifically accuse anyone else. Oh, and he has a box with a dollar sign on his back.
Note that figuring out which one is truthful isn't required, as the culprit is one of the liars. For the record, it was probably O'Cantler. He accuses one of the others of lying, but not of taking the money.
The Homestar Runner game Where's An Egg is based on this, only with 9 knights or knaves and a limited set of questions you can ask. Oh, and it's in Russiannote The main gameplay is all in pictures, making the game easily playable regardless of the language you speak. The Russian is just the framing device.
Spoofed in Chicanery, where Ness rants at length about how overused this device is after getting it from one of the doors in the gang's new secret lair. He even cites the use of this trope in Labyrinth: "Now you've made me think of David Bowie again. Thanks loads."
Pokey:(Hacking at one of the doors with a sword) Does this hurt?
Door: AAAAAAGGHH! AAAAIEEE!!! NOT IN THE LEAST!
XKCD also has fun with it here, as quoted above. The Alt Text takes it further: "And the whole setup is just a trap to capture escaping logicians. None of the doors actually lead out."
In Episode 327 of The Order of the Stick, this is the Test of the Mind the Order goes through to get to the Oracle of Sunken Valley (which prompts Roy to remark "that's the last nail in the coffin for the hope that these Tests would be even remotely original"). Haley solves it by shooting one of the guards, then noting that the guard she shot is screaming "you shot me!" while the other guard insists "she totally didn't shoot you". The next time Haley passes through, the guards remember her and hastily direct her to the correct path before she can do anything. Haley doesn't understand why, because the Oracle's memory charm means nobody remembers anything that happens in the valley except for the answer he gives them.
Girly features what probably is the most nonsense solution for the problem in the strip "Knights and Knaves". Basically, the right question is "Are you wearing a sombrero?" Of course, given the setup of that particular instance ("the correct path lies with the one who tells the truth"), any question you already know the answer to will do.
Parodied in Partially Clips: here. No solution is offered or expected, but for the record, the puzzle is unsolvable, as the premise is false - both the second and third heads contradicted themselves, something which only the alternator would do.
In this strip of Nobody Scores!: Jane simplifies the problem by opening both doors and shoving the knight and knave through them.
On Bob and George, when Mega Man gets to Gemini Man, one of them claims that they tell riddles (they don't) and begins with this one. When the other tries to protest, the first merely passes off everything as a lie. Mega Man just stands there, reflecting on what Wily bots have been reduced to.
Ricky and Steve do the "Heaven and Hell" version of this with Karl Pilkington on The Ricky Gervais Show. His answer is to pretend to be a postal worker and ask them to send God out to sign for it.
Rather hilarious bit of Fridge Logic is the fact that they use the "Hell-Door guard lies, while the Heaven-Door guard tells the truth" version, meaning that their answer isn't that much better, being overly complicated.
JourneyQuest does this with Glorion killing the truth-telling gargoyle, believing the liar, and getting annoyed by the liar contradicting him — finally asking if he wanted to die. The liar, forced by his nature, says yes, and is thrown through the door he has convinced Glorion leads to his death... demonstrating its safety.
This is one of the many puzzles presented to The Powerpuff Girls by Him in the episode "Him Diddle Riddle." Blossom uses the "If I asked the other person..." variant.note Real Keane (Call here R) tells truth, Fake Keane (Call her F) lies. Blossom asks who the other Ms Keane would say is real. One says "She would say she was real." The other says "She would say I was real." Both would obviously claim to be real, but F would say that R would say that F is the real one She then tries to explain the whole thing to Bubbles and Buttercup, whose reactions could be summed as X.X faces. However, if you do follow the explanation, you find she used the right logic, but picked the wrong one, even though her explanation implies that she knew which one to pick.
Also subverted in an episode of Samurai Jack. A two-headed creature poses this riddle to Jack, claiming that one of his heads is magic, and if he chooses to be swallowed by it, he will be granted a wish, while if he is swallowed by the other head he will simply be eaten. One head always lies, and the other always tells the truth. Jacks solves the riddle using the "If I asked the other one which was correct..." solution, but it turns out that it was all just a trick by the creature to get idiots to willingly feed themselves to it.
There was a brain teaser in a Doctor Who annual about two captured soldiers (astronauts?) who were told that they could make one statement, If their statement was judged as true they would die by lethal injection, if their statement was judged as false they would die by hanging. They managed to make a single statement that meant the judge had to let them go.
The answer? They make the statement "I will die by hanging" if they hang them that makes the statement true, which should mean the die by lethal injection, which would then make the statement false, which would mean they should die by hanging and so on.