Problem:

At a crossroads, one road leads to a town where everyone tells the truth, and the other to a town where everyone lies. There is no sign to indicate which is which. You meet a man from one of the towns at the crossroads. What single question can you ask him to find the way to the town of truth-tellers?

Scroll to the bottom of the page for the answer

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Solution:

Ask the man, "If I asked you which way is the town of truth-tellers, what would you say?" This forces a truth-teller to point to his town, and a liar to lie about lying, thus also pointing to the town of truth-tellers.