You are in a room with 2 doors and 2 guards. One of the doors will guide you to freedom and behind the other is a certain doom --you don't know which is which. One of the guards always tells the truth and the other always lies. You don't know which one is the truth-teller or the liar either. You have to choose and open one of these doors, but you can only ask a single question to one of the guards. What do you ask so you can pick the door to freedom?

Answer: If you asked the truth-guard, the truth-guard would tell you that the liar-guard would point to the door that leads to death. If you asked the liar-guard, the liar-guard would tell you that the truth-guard would point to the door that leads to death. Therefore, no matter who you ask, the guards tell you which door leads to death, and therefore you can pick the other door.

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