You stand before two doors. One leads to certain doom, and the other leads to freedom. Two guards stand beside them: one always tells the truth, and the other always lies. You do not know which guard is which, nor which door is which. You are permitted to ask just one guard a single yes-or-no question. What question do you ask to guarantee your freedom?
Ask either guard: 'If I were to ask the other guard which door leads to freedom, which door would he point to?' Then choose the opposite door.
Both guards will point to the doom door. If you ask the truth-teller, he knows the liar would point to doom, so he truthfully points to doom. If you ask the liar, he knows the truth-teller would point to freedom, so he lies and points to doom. Thus, the indicated door is always wrong.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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