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 re(5): Solution - WRONG ANSWER, NEW PUZZLE
We've lost the point now. There are only two solutions. Look at the difficulty rating on this one, then ask yourself why you're beating yourself up about it. Fletch's solution is logically correct, but the answer I always heard when I was a kid was to ask either one "What would your brother say if I asked him where the treasure is?"
Both are true, it boils down to which one Gamer thought of.