You're on a crossroad. One path leads to the left, one to the right.
There is no going back.
Each path takes you somewhere. One of them takes you to the Truth, salvation, and the other one into failure and destruction. You, of course want to get to the Truth. You don't know which path is which.
By each path, there is a man standing. One man always tells the truth while the other one always lies.
You can only ask one of them a single question, in order to reveal which path is the one you wish to take (the Truth one).
What do you ask him?