Two guards stand before two doors: one leads to eternal freedom, the other to certain doom. One guard always tells the truth, and the other always lies. You do not know which is which, nor which door is which. You are allowed to ask just one guard a single 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.
This is the classic knight-and-knave paradox. Asking this compound question forces either guard to point toward the doomed door, allowing you to safely pick the other one.
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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