Riddle: There are two doors with faces on them. One door always tells the truth, and one door always lies. Also, one door leads to heaven and one to hell. You do not know which one is which or where each leads. You can only ask one door one single question to find out which door leads to heaven. But remember, you don't know if it is lying or not. What is the question?
Answer: "If you were the liar, would you tell me this door leads to heaven?" (You choose the opposite. The liar obviously lies, and the truth-teller speaks as if he were the liar, so he also "lies.")