Classic Riddle:
You come to a fork in the road. There are two men, one from each road. One leads to heaven, one leads to hell. One man always tells the truth, the other always lies. You are allowed to ask 1 question and 1 question only. What do you do?
Correct Answer:
Ask them which way is heaven (of course they each point to their road).
Punch one of them in the face. If he says it hurts or that he didn't expect that, go down his road. If he starts saying it didn't hurt at all, go down the other one's road.
Ontopic: The above is a horrible solution to the riddle. I could come up with a way better one.