Bob died, and met God and the Devil in his afterlife. He knew that God always spoke the truth and the Devil always lied. He did not know which one was which. He was allowed to ask one yes-no question which both would answer. If he succeeded in deducing the identities of the two entities, he would be sent to heaven. Otherwise, he would be sent to hell.
Bob had other plans. He wanted to have some fun. He asked a question that stumped God and the Devil. They could not answer it even though they knew the facts required to answer the question.
Impressed with his logical prowess, they decided that Bob would work with them for the rest of his afterlife. What question might he have asked to stump God and the Devil?