In a small town, there is a mysterious room with two doors. One door leads to certain death, and the other door leads to freedom. In front of each door, there is a guard. One guard always tells the truth, and the other guard always lies. You don't know which guard is which, and you don't know which door leads to freedom.
You are allowed to ask one guard one question to determine which door leads to freedom. What question do you ask?
Solution:
Ask either guard, "If I were to ask the other guard which door leads to freedom, what would he say?" Then, choose the opposite door of the guard's response.
Explanation:
If you ask the truthful guard, he will truthfully tell you what the lying guard would say. If you ask the lying guard, he will lie about what the truthful guard would say. In both cases, the answer will guide you to the door that leads to certain death. Therefore, by choosing the opposite door, you ensure you are heading towards freedom.
