Math & Logic · Ages 18+
Riddle #209
You reach a fork in the road: one path leads to the city, the other to certain ruin. Two guards stand there — one always tells the truth, one always lies, and you cannot tell which is which. You may ask exactly one yes-or-no question, to exactly one guard. What do you ask?
You solved this one before
Open-ended riddle: think it through, then reveal the answer.
Answer: Ask either guard: "If I asked the other guard whether the left path leads to the city, would he say yes?" Then take the opposite of the answer you get.
Either the truth-teller faithfully reports the liar's lie, or the liar misreports the truth-teller's honest answer — either way the reply is inverted exactly once, so it is reliably wrong. Reliably wrong is as useful as reliably right.