Personal Comment: This one has many variations. This is the one I like.
The Liar & Truth-teller Enigma!
What we know: A traveller comes to a fork in the road and does not know how to get to his destination. 2 men are at the fork in the road. 1 of them always tells the truth, and the other one always lies. He may ask the men on question to find his way.
Enigma: What question does the man ask these men?
Solution: He asks them, "If I ask the other man which way to go, what would his answer be?" They would both answer in the same way. Whatever the answer is, go the other way.
If the man you ask always tells the truth, then the other always lies. The man you ask will tell truthfully that the answer would be the wrong way.
If the man you ask always lies, then the other man tells the truth. The man you ask will lie and tell you that the other man would tell you the wrong way. In both cases the answer you would receive would be the wrong way. Go the other way.
