History Main / KnightsAndKnaves

* Subverted in ''Anime/YuGiOh'' During the Duelist Kingdom arc, the Paradox Brothers confront Yugi and Jounouchi/Joey with this puzzle. Joey thinks he's solved the riddle, seeing that he's heard it before, but 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 (they both say that one ''always'' lies and one ''always'' tells the truth, which is impossible because someone who ''always'' lies would be unable to give an honest description of the puzzle, and thus could ''never'' agree with someone who always tells 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 [[BeatThemAtTheirOwnGame tricks them]] by making them think he's choosing one door via ExactWords, waiting for their answer, and revealing he chose the other one. [[spoiler:Both doors lead to the exact same place anyway]].

* ''VideoGame/DarkSeed II'' 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.

* The ''Series/DoctorWho'' serial "[[Recap/DoctorWhoS13E3PyramidsOfMars Pyramids of Mars]]" features 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 SufficientlyAdvancedAlien was protected only by ''[[SolveTheSoupCans 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.

* In ''Machinima/Supermarioglitchy4sSuperMario64Bloopers'', [=SMG4=] and Mario encounter two Piranha Plants in [=SMG3=]'s dungeon filling this role. Naturally, [=SMG4=] solves the riddle with the opposite answer solution, while Bowser just sets them both on fire and enters the wrong door.

* These logic problems have been extensively studied. There are many variants, such as zombies who always lie, and humans who always tell the truth, that may be sane (believe only true things) or insane (believe only false things). An insane zombie always wants to lie, but believes untrue things, and thus tries to lie based on inaccurate beliefs. For example, an insane zombie asked if he is a zombie believes he is a human, but tries to lie, so an insane zombie says, "I am a zombie."

->''And over there we have the Labyrinth guards. One always lies, one always tells the truth, and one stabs people who ask tricky questions.''

[[quoteright:328:[[Webcomic/TheOrderOfTheStick https://static.tvtropes.org/pmwiki/pub/images/Knights_and_Knaves_8882.png]]]]

* ''[[VideoGame/DarkSeed Dark Seed II]]'' 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.

* Webcomic/TheBestGamepiecePhotocomic: [[https://www.theduckwebcomics.com/The_Best_Gamepiece_Photocomic/5642716/ This strip]] fearutes a character using a LogicBomb against one. The guards respond by [[spoiler:[[CuttingTheKnot just telling him the answer.]]]]

* Subverted in ''Anime/YuGiOh'' 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 (they both say that one ''always'' lies and one ''always'' tells the truth, which is impossible because someone who ''always'' lies would be unable to give an honest description of the puzzle, and thus could ''never'' agree with someone who always tells 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 [[BeatThemAtTheirOwnGame tricks them]] by making them think he's choosing one door via ExactWords, waiting for their answer, and revealing he chose the other one. [[spoiler:Both doors lead to the exact same place anyway]].

* In Werner Herzog's ''Film/EveryManForHimselfAndGodAgainstAll'', 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."

* Sally Acorn of the ''ComicBook/SonicTheHedgehog'' comic solved one of these in the "In Your Face" special.

* ''VideoGame/SonicTheHedgehog2006'' 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 [[spoiler: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]]. FTA of LetsPlay/HellfireCommentaries reacted with a very understandable AtomicFBomb.

'''G2''': jesus christ, Daniel, I said I was sorry.\\

[[caption-width-right:328:[[CuttingTheKnot She hates riddles.]]]]

* ''Website/SCPFoundation'': [[http://www.scp-wiki.net/scp-4390 SCP-4390]], a knock-off of ''Film/{{Labyrinth}}'', features a variant on this puzzle with two Spanish-speaking brothers named Señor Honestidad and Señor Deshonestidad as one of the challenges the exploration team faces. [[spoiler:The one asking the questions bungles the question by asking them which path to go down, but their names are a dead giveaway]].

* ''TabletopGame/Warhammer40000'': Kairos Fateweaver, a Lord of Change with two. heads, knows everything, but when asked a question, one head gives the correct answer, while his other head gives 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...

Heroes who have [[WhenAllYouHaveIsAHammer neither the patience nor aptitude]] for logic puzzles generally just [[CuttingTheKnot skip straight to the violence]] when confronted with this one. Of course, the puzzle was "meant" for [[NonActionGuy people for whom a pair of armed guards are a formidable obstacle]], rather than for your standard ActionHero (if the guards aren't monsters, of course). In video games, it can also be brute-forced by SaveScumming.

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 the guard says "yes", then you go through his door; if he says "no," you go through the other guard's door. 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 one of the guards "Which door would ''the other guard'' say is the correct door?" They will both give the same answer; whichever door they indicate, that's 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.

* A similar situation exists in ''TabletopGame/{{Warhammer 40000}}'' 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 gives 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...

* The LicensedGame of ''Film/{{Labyrinth}}'' for the Commodore 64 includes the scene as described in the Film folder; but as the engine was too limited to let you ask specific questions, the solution is different. The player must open each door and see which path has the sign saying "[[NeonSignHideout To the Castle]]" and which says "[[ThisWayToCertainDeath To Certain Death]]." The real danger is assuming that [[OutGambitted something so obvious must be a trick]] and falling to your doom.
* The DungeonTown of Zozo in ''VideoGame/FinalFantasyVI'' is part of a Knights and Knaves puzzle where ''everyone'' in town is a Knave. There are people there that don't lie, but none of them are native to the town, and they don't contribute to the puzzle. The Knaves all tell you what time it is, and each statement is false. By process of elimination, you can find out the correct time, and use a clock late in the dungeon to access the {{Chainsaw|Good}} for Edgar.
* Featured/spoofed in the browser-based {{MMORPG}} ''VideoGame/KingdomOfLoathing''. In an early version of the final quest, you had 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 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: [[spoiler: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 ''VideoGame/{{Ultima}}'' features a two-headed horse called the [[Literature/DoctorDolittle 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 MoneySpider enemies actually make the "wrong" answer more attractive.