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Let's say that you are a prisoner who was suddenly given the right to go free, but only if you cope with such a task: before you are two doors, one of them leads to freedom, the other - the road to death. Two guards sit, one of them being a liar, and the other always telling the truth; You do not know which one is who. You should, having asked only one question to one of the guards, determine the road to freedom. What question do you ask?

Answer: There are an infinite number of solutions, but the most beautiful of them are three:
- Showing on the concrete door: "Your friend would say that this door leads to freedom?" The answer "yes" means that this door does NOT lead to freedom.
- "Is there a guard who speaks the truth before the door leading to freedom?" The answer "yes" means that you need to enter the door next to which the guard is sitting, to whom you asked a question.
- Showing on the concrete door: "If I asked you if this door leads to freedom, what would you say?" The answer "yes" means that this door leads to freedom. This answer is suitable even when there is no second guard.