You are trapped in a dungeon, and see two doors in front of you. One is a way out, andther will lead to certain death.
Two people are standing in front of these doors, one of them a Knight, another - a Liar. Naturally, you do not know who is who, but they know each other's true nature.
Both the people know which way is correct, but you can only ask one question of one of them before making your choice.
What will you ask?
(In reply to No Subject
by K Sengupta)
Another solution would be point to a dorr and to ask each of the persons the following question, "If I asked your companion, if that door leads to freedom, would he say "yes""
If the door is indeed the correct one, both the individuals would respond as "yes", otherwise the reply of both of them would be "no".