You walk up to a mountain that has two paths. One leads to the other side of the mountain, and the other will get you lost forever. Two twins know the path that leads to the other side. You can ask them only one question. Except! One lies and one tells the truth, and you don't know which is which. So, What do you ask?
To explain, one of the twins lied and one told the truth. The one who lied would say what the other twin said (The truth) and vice versa. Then, you will know which one is the right path!