Riddle: You are a prisoner in a room. There are two doors in this room. One leads to freedom, one leads to death. Each door is guarded by a guard. One always lies, one always tells the truth. You don't know which is which. What single question can you ask to find the door to freedom?
Answer: "If I were to ask the other guard which door leads to freedom, what would he say?" Then you choose the opposite one. Because if you ask the one that tells the truth, he would say the one that leads to death. The lying one will also say the one that leads to death, because he lies.