A detective was walking to the scene of the crime, when he came upon a fork in the road. One either side of each of the roads, stood two men. One man only told lies, and the other man only told the truth. The road that would lead the detective to the crime had the man who only told lies on it. The detective needed to find out which man only told lies, and which man only told the truth, with only one question.
What was the question the detective ask?
He asked the men something he already knew. A few examples would be: "Is water wet?", "Are rocks hard?", and "Does a sponge have holes in it". The man who only told lies would say the wrong answer, and the man who only told the truth would say the right answer.