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?

Ask the brothers "Does Bob Lie?"

If Bob is the liar he will say NO
If Bob is not the liar he will also say NO
If the other brother is the liar he would say YES
If the other brother is not the liar he would say YES.

So whoever you ask doesn't matter. If the answer is No then it is Bob, if the answer is YES than the other brother is BOB

Posted by Ken on 2002-09-11 20:26:22