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 one yes-or-no or simple question to one guard.
What question do you ask to find the door to freedom?
💡 Hint
Ask one guard what the other guard would say.
Reveal answer
Ask either guard: “Which door would the other guard say leads to freedom?” Then go through the opposite door.
Suppose you ask the truth-teller. He honestly reports what the liar would say, and the liar would point to the dead end. So you hear the dead end. Now suppose you ask the liar. The truth-teller would point to freedom, but the liar lies about that and points to the dead end. Either way, the door you’re told is the dead end, so you take the other one. The question works because the answer always passes through exactly one lie.
Did you get it?
About This Puzzle
This is one of the most famous logic puzzles ever written, and it still beats most people the first time. You only get one question, so make it count.
How to Think About It
Asking “Which door is the exit?” doesn’t help, because you don’t know whether you’re hearing the truth. You need a question whose answer comes out the same no matter which guard you ask. Try to make the answer pass through both guards: one truth and one lie together always make a lie.
🧠 Up nextThe Rooster on the Barn Roof→