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?
One of the brothers should be asked this question: "Is Bob a knight?"
If the person is Bob, then irrespective of whether Bob is a liar or a knight, he will answer "Yes".
If that person is NOT Bob, and Bob is a knight, then that peron will lie and say "No" as a response.
If that person is NOT Bob, and Bob is a liar, then that person will truthfully offer "No" as a response.
Consequently, if the response to the query is "Yes", then the person is Bob and, if the answer is "No", then the person is Bob's brother.
Puzzle Solution
