You are on the road to heaven and you reach a fork in the road. One of the paths leads to heaven, however the other path leads to hell. At the fork there are two gods, one who always tells the truth, and one who always lies. You are allowed to ask one question to one of the gods.
What question do you ask to determine the path to heaven?
Ask one of the gods “If I were to ask the other god which path leads to heaven, what would he say?” Then take the opposite path.
Proof
First let us consider the question “Which path leads to heaven?”
To this the truthful god will point to the path to heaven, and the lying god will point to the path that leads to hell.
Now if we look at the question given in the solution, the truthful god will answer honestly and point to the path to hell (which is what the lying god would say leads to heaven). The lying god will also point to the path that leads to hell, because the it is the opposite of the path that the truthful god would say leads to heaven.
Thus whoever you ask, they will point to path that leads to hell. We simply pick the other path.