One of the most famous riddles about guardians who tell the truth or lie. The usual trap is to try to figure out who is who before asking. There is a more elegant way.
The guardians of the two doors
Riddle statement
You are faced with two doors: one leads to the exit and the other to perdition. Next to them are two guardians. One always tells the truth and the other always lies, but you don't know which is which.
You can ask one of the guardians a single question. What questions to safely choose the right door?
Show solution
Solution
Ask either of them:
"If I asked the other guardian which door was correct, which door would he point to?"
Then choose the opposite door.
If you ask the truthful guardian, he will tell you honestly what the liar would answer. The liar would point to the wrong door, so the truthful one will also point you to the wrong door.
If you ask the lying guardian, he will lie about what the truthful one would answer. The truthful person would point to the correct door, but the liar will point out the opposite: the wrong door.
In both cases the answer points to the wrong door. Therefore, you must choose the other.