Wasn`t it Whistler who said that a great painter was one who could hide the effort which he put into his work? The same thing goes for gags. It often takes hours to think up something that is said in seconds.
I can`t give you a definition [of humor]. It is too subtle to be pinned down. I can say that it differs from wit, which exaggerates the truth, while humor presents the truth in an original way.