Puzzles
The liar, the truth-teller, and two doors
Two doors: one leads to freedom, one to doom. Two guards: one always tells the truth, one always lies — you don't know which is which. You may ask one guard a single question. What do you ask?
Reveal the answer
Ask either guard: 'Which door would the other guard say leads to freedom?' — then take the opposite door. The truth-teller truthfully reports the liar's wrong answer; the liar lies about the truth-teller's right answer. Both point to doom, so you walk the other way.
— Raymond Smullyan, What Is the Name of This Book? (knights-and-knaves puzzles) — 1978
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