I’ve only heard it with one question, that’s the whole point. Otherwise you just ask a guard some trivial question (e.g. What color is the sky?) to determine which is the liar, then just ask which is the safe door.
The whole point is to get the information you need from a single question.
Ah. Normally I see this with no limit on questions. You’re right. It’d only work with at least two questions.
I’ve only heard it with one question, that’s the whole point. Otherwise you just ask a guard some trivial question (e.g. What color is the sky?) to determine which is the liar, then just ask which is the safe door.
The whole point is to get the information you need from a single question.