An easy, well known riddle.
There are two doors.
One of them leads to Heaven, the other one to Hell.
One of them always speaks the truth, the other one always lies.
However, you cannot know which is which, for they both are completely alike.
In order to choose the correct door that might lead you to Heaven, you are given the chance to ask them a single question, the same question both shall answer.
What would you ask them to find out which one is the correct door?