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?
(In reply to
Solution by Penny)
... Any answer from Twin A other than the true location, would be the truth, and he is a Liar. So Twin A would have to reveal the actual location. ...
This would work if there were just 2 possible locations for the treasure. Let me explain...
Imagine 4 possible locations for the treasure w,x,y,z. And lets assume that the treasure is at location z. Now Twin B, if asked directly, would tell you either of w,x,y.
Twin A knows this. (And possibly the way his twin brother's fibbing patterns). Twin A could tell you either of w,x,y and still be lying.

Posted by Syzygy
on 20050705 10:01:54 