You are traveling in a land where every inhabitant either always lies or always tells the truth. You come across two caves: in one lies treasure and in the other lies certain death. There is a guard in front of the caves, and a sign announcing that the guard will answer one yes/no question to any traveler.
You don't know whether the guard tells the truth or lies - what single yes/no question should you ask the guard if you want to find the treasure? (Or try to show that it is impossible for you to determine which cave has the treasure.)