A traveler finds himself on an island inhabited by two opposing tribes. Members of one tribe must always tell the truth, while members of the other always lie. He comes to a fork in the road and has to ask a bystander which path he should take to reach a nearby village. The traveler has no way of finding out whether the bystander is a liar or a truth teller. What question should he ask to determine which path to take?
The question is answerable with a yes or a no.
The traveler points to one of the roads and asks, "If I were to ask you if this road leads to the village, would you say yes?"
If the road does lead to the village, the liar would have said no to the direct question, but as the traveler's question was phrased, the bystander lies and says he would have responded yes.