Liar puzzles are a type of recreational logic puzzle. The general scenario is that, in some far-off land, there are two groups of people: truth-tellers, who always tell the truth, and liars, who always lie. The classic liar puzzle is the following:
A traveller, travelling in a foreign country, reaches a fork in a road. In one direction lies a village where all of the residents always tell the truth, and in the other lies a village where all of the residents always lie. The traveller would like to visit the truth-tellers' village, but does not know which way to go. There is a man from one of the villages at the fork, but the traveller does not know which village the man is from. The traveller asks the man one question and, from his answer, determines which road to take. What question did the traveller ask?
Answer: The traveller asks "Which of these two roads leads to your village?" If the man is a truth-teller, he will point to the truth-tellers' village. If the man is a liar, he will lie and point to the truth-tellers' village. Either way, the traveller simply goes in the direction that the man pointed.
See also: lie detection puzzles, metapuzzles.