Two Guards and Two Doors Riddle: The Question to Ask
Classic logic puzzle about truth and lies
The riddle
Two doors lie before you: one leads to freedom, the other to a trap. Two guards stand in front of them; one always tells the truth, the other always lies, and you do not know which is which. You may ask only one question, to only one guard. What question should you ask to find the door to freedom?
Need a hint?
Think about including the other guard in your question, and about what each of them would do with the other's answer.
Show the answer and the solution
Answer: Ask one of the guards: “If I asked the other guard which door leads to freedom, what would he answer?” Then take the other door, the one not indicated.
The reasoning
The truthful guard and the liar both know which door leads to freedom. If you ask the truthful one what the liar would answer, he faithfully reports the wrong door the liar would point to. If you ask the liar what the truthful one would answer, he lies about the door the truthful one would point to, and again points to the wrong door. In both cases the answer points to the wrong door, so you simply take the other one.
What next?
Reasoning improves like memory: by practising often. Next, test yourself on our general knowledge quizzes.