Riddle: There are two perfectly the same doors, one that leads to paradise, the other leads to death. There are also two men, one must always lie, and the other must always tell the truth. You may ask one question and only one single question to the two men to help you figure which door is which. What is the one question that will tell you which is which?
Answer: What door would the other man pick? (the man who tells the truth would point to the wrong door and the man who lies would point to the wrong door too because the man who tells the truth would have pointed to the correct door, so they would both point to the wrong door so you pick the opposite one)