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?

This is a duplicate of question 92321 - the classic two guards logic puzzle. The optimal question is 'Which door would the other guard say leads to hell?' because it forces both the truth-teller and liar to point to the same door. The liar lies about what the truth-teller would say, while the truth-teller truthfully reports what the liar would say - both identify the heaven door.