You are to meet with two brothers. You know that one of them, named Bob, has a very important package for you. Unfortunately, you never saw Bob or his brother, and don't know who is who.

To make the matter worse, one of the brothers is a pathological liar who never tells the truth, while the other never lies. Not only do you not know which is the liar and which tells the truth, but also, you do not know wether Bob is the liar or not.

Upon meeting the brothers, you can ask one short question of either of the brothers (but not both) to establish with certainty which of the two is Bob. (It is possible to do this with a question of four to five words.)

What question would you ask?

(In reply to the answer by kristina)

That wouldn't work, since you do not know which of the two brothers is Bob. (That's actually the point of the problem.) All such a question would tell you, is wether or not the brother that you asked is a liar. But you would still have no way of knowing if you are talking to Bob or to his Brother.

Posted by levik on 2002-08-27 03:47:57