You’re in a prison with 2 doors. One leads to freedom, the other to execution. There are 2 guards, one by each door. One guard always tells the truth, the other always lies. You don’t know which is which. You can ask one question to one guard to determine the door to freedom. What do you ask?
Answer
'If I asked the other guard which door leads to freedom, what would he say?' Then choose the opposite door.