This is based on a story of two doors and two guards, one who lies and one who doesn't. One door leads to heaven and one to hell, and the puzzle is to find out which door leads to heaven by asking one of the guards a question. What question should be asked?

In this classic logic puzzle with one truth-teller and one liar, asking 'Which door would the other guard say leads to hell?' forces both guards to point to the same door - the door leading to heaven. The truth-teller truthfully reports what the liar would say (the liar would falsely claim hell leads to heaven), while the liar lies about what the truth-teller would say. Both end up identifying the heaven door.