You are on a treasure hunt, wondering where the "Truth Treasure" is buried. You have no idea where it is buried, so you go Truth Town (made up of only Liars and Knights) because it is famous for holding the treasure's secret location.
When you enter the city, you see two twins and they know each other's types, but you don't. One twin says "(Exactly) one of us is a Knight..." and the other twin says "...and (exactly) one of us is a Liar." You can only ask one question and only to one person. You only care about where the treasure is kept, and obviously a liar would give you a wrong answer, so what should you do?
Since they have claimed that there is at least one liar, they cannot both be knights (knights wouldn't lie about being liars).
It is impossible for one to be a knight and the other a liar, because then they would both have told the truth and neither could be the liar.
They must therefore both be liars.
Either one of them, if asked about the location of the treasure will give any answer that is not where the treasure is.
Each man know's this about his brother, but doesn't know what exact lie he will tell. If asked the question: "Where would your brother tell me to find the treasure?" The only answer that he can be sure to be a lie is the True location of the treasure.
Posted by scott
on 2005-08-01 14:12:58