There are two doors and there is one guard for each door. One door leads to life and the other leads to death. You have only one question to ask and you can only ask one guard. One of the guards always tells the truth and the other always lies. What question would you ask to find out which door is the door to life?
You would ask: "If I were to ask the other guard which door leads to life, what would he say?" The guard that tells the truth knows that the other guard would lie. So he tells the truth and points to the door that leads to death. The guard that lies knows that the other guard would tell the truth. So he lies and points to the door that leads to death. Both guards point to the death door.