You stand before 2 doors. One leads to freedom, the other to a trap. Each door has a guard. One guard always tells the truth and the other always lies, but you don’t know which is which. You may ask 1 guard 1 question. What do you ask?

Ask either guard: “Which door would the other guard say leads to freedom?” Then take the other door.

Ask either guard: “Which door would the other guard say leads to freedom?” Then walk through the other door.

If you ask the truthful guard, he honestly tells you the liar’s answer, which points to the trap. If you ask the liar, he lies about the truthful guard’s answer, which also points to the trap. Either way, the door they point to is the wrong one, so you take the other.

Why it’s fun

You try to figure out who is lying. You don’t need to. The right question makes both guards point the same way.