You die in a plane crash and find yourself in front of two identical doors. One door leads to heaven and the other to hell. There are two identical angels at each door. One angel can only tell the truth, while the other angel can only lie. Only the angels themselves know who is lying and who is not. You can only ask one question to one of the angels. What is the only question that will get you to heaven?
Would a second angel tell me that this door leads to heaven? If the asked angel turns out to be the one who always tells the truth and the indicated door turns out to be heaven, he will answer: No. If the asked angel turns out to be who is lying, he will also answer No. If we hear the answer “No”, we enter the indicated door. In turn, if the indicated door turned out to be hell, each of the angels replied: Yes. Then we enter the second door.
