Riddle: You are in a room with 2 doors -- one leads into the dungeon, one leads to freedom. There are two guards in the room, one at each door. One always tells the truth. One always lies. What one question can you ask one of the guards that will help you pick the door to freedom?
Answer: If you just wanted to determine which one tells the truth and which lies, you would ask: "If I were to ask the other guard whether you always tell the truth, what would he say?" A reply of "no" means you are talking to the truth teller, a reply of "yes" means you are talking to the liar. So since you only have one question to select the right door, you would have to ask "If I were to ask the other guard, which door would he say leads to freedom?" You would then select the door opposite to the one he picked.