Explain xkcd: It's 'cause you're dumb.
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This comic alludes to a famous Knights and Knaves logic puzzle, and specifically to the version featured in the Jim Henson movie Labyrinth, with two doors and two guards. One guard always lies, and the other always tells the truth. One of the doors leads to freedom, and you can only ask one guard one question. The solution to this riddle involves a tricky question indeed, and in the xkcd-version one would risk a stabbing from the third guard. If you want to give the original puzzle a try for yourself, don't read the spoiler in the next paragraph.
Solution 1: Ask one guard (it doesn't matter which one) which door the other guard would say leads out. The door indicated does not lead out.
Solution 2: Ask any guard what answer he would give if you asked him the question "Which door leads out ?". The door indicated leads out.
- [Three guards with spears stand in front of three doors. Black Hat and Cueball stand in front of the guards.]
- Black Hat: And over here we have the labyrinth guards. One always lies, one always tells the truth, and one stabs people who ask tricky questions.
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Just ask which color is the sky.. 184.108.40.206 (talk) (please sign your comments with ~~~~)
- Oh, although the strip doesn't explicitly say so; in those riddles you can normally only ask one question. --St.nerol (talk) 23:00, 27 January 2013 (UTC)
- There's another (more traditional) three-guard variation where one guard always tells the truth, one guard always tells a lie and the third alternates between pure truth and pure lie (and you don't know which flip they're currently flopped upon). But you still only get to ask one question of one guard. Have fun with that one. My personal solution certainly has a degree of convolution, but I've heard other workable answers. 220.127.116.11 02:24, 21 June 2013 (UTC)
- @18.104.22.168, you would know which one lies but you would not know which door leads out. Tharkon (talk) 23:13, 10 October 2013 (UTC)
- Eh, well, even if you had a perfect question to ask in this case, a lot of good would that do you: it'd only reveal the truth behind the setup, that none of the doors lead out. :p -- 22.214.171.124 08:20, 8 November 2013 (UTC)
- Well yes it says that in the title-text. But good pick-up. 126.96.36.199 02:31, 6 February 2014 (UTC)
One question, of one guard. I really like the original form of this riddle. It's a bit of a trick, though. It is crucial that the guards "know" each other's rules, but this is not even implied. And if it was stated in the question, that would probably be a good enough clue to get you to the answer. Of course, once you know the answer it seems trivial, but I wonder what percentage of people actually worked it out for themselves? Another good one is Monty Hall, even though that is pure, straightforward probability. 188.8.131.52 18:11, 17 January 2014 (UTC)
- I think somebody needs a hug! 184.108.40.206 18:11, 17 January 2014 (UTC)
The whole problem with this entire riddle is that if they are both liars you are screwed! Nothing in the riddle establishes a fact that they aren't liars. Now if there was a known truth teller in the riddle that explains the nature of the guards or the narrator does it, then the above solution works. 220.127.116.11 (talk) (please sign your comments with ~~~~)