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A boy was walking down the road, and came to a place where the road divided in two, each separate road forking off in a different direction.
A girl was standing at the fork in the road. The boy knew that one road led to Lieville, a town where everyone always lied, and the other led to Trueville, a town where everyone always told the truth. He also knew that the girl came from one of those towns, but he didn't know which one.
Can you think of a question the boy could ask the girl to find out the way to Trueville?
If the girl is from Lieville, she will point to Trueville because she has to lie; if she is from Trueville, she'll point to Trueville because she has to tell the truth. So whatever road the girl points to will be the road to Trueville.