Riddle: There is a fork in the road. You can go right, or you can go left. There are two sisters at the fork. One always lies, and one always tells the truth, but you don't know which sister is which. You have one question to ask the sisters. What do you ask?
Answer: What way would your sister say to go? (The one who tells the truth would say that her sister would lie and say the wrong way. The one who always lie would say that her sister would lie and say the wrong way. Then you would go the other way.)