Two Guards, One Question

Overview

Popularised by the film Labyrinth, this riddle asks for one question that works regardless of which guard you happen to ask.

Ask either guard: "Which door would the OTHER guard say is safe?"
The truth-teller honestly relays the liar's wrong answer; the liar lies about the truth-teller's right answer.
Both routes point to the dangerous door, so pick the other one.

Embedding one guard's answer inside the other guarantees the reply passes through exactly one lie — so it's always false, no matter whom you ask.

Smullyan's harder versions add a third 'random' guard, solved with self-referential questions.
The trick — force the answer through an odd number of negations — recurs across logic puzzles.

Yes — ask what the other guard would say is dangerous, and follow that door. Both framings are equivalent.

Popularised by the film Labyrinth, this riddle asks for one question that works regardless of which guard you happen to ask.

Ask either guard: "Which door would the OTHER guard say is safe?"
The truth-teller honestly relays the liar's wrong answer; the liar lies about the truth-teller's right answer.
Both routes point to the dangerous door, so pick the other one.

Embedding one guard's answer inside the other guarantees the reply passes through exactly one lie — so it's always false, no matter whom you ask.

Smullyan's harder versions add a third 'random' guard, solved with self-referential questions.
The trick — force the answer through an odd number of negations — recurs across logic puzzles.

Yes — ask what the other guard would say is dangerous, and follow that door. Both framings are equivalent.