You find yourself in a room with two doors. One door leads to freedom, and the other door leads to a room full of deadly traps. There are two guards, one in front of each door. You know that one guard always tells the truth, and the other guard always lies. You don't know which guard is which, nor do you know which door leads to freedom. You can ask only one question to either guard to determine the door that leads to freedom. What question do you ask?
You can ask either guard the following question: "If I were to ask the other guard which door leads to freedom, what would he say?" Now, let's examine the possible scenarios:
(1) If you ask the truthful guard, he will tell you that the lying guard would point you to the door with the deadly traps.
(2) If you ask the lying guard, he will also lie about what the truthful guard would say, and he will still point you to the door with the deadly traps.
In both cases, the guard you ask will point to the door with the deadly traps. Knowing this, you can safely choose the other door, which leads to freedom.
