Found via the Onion amicus brief, attributed to Mark Twain at this source:

“The humorous story is told gravely; the teller does his best to conceal the fact that he even dimly suspects that there is anything funny about it; but the teller of the comic story tells you beforehand that it is one of the funniest things he has ever heard, then tells it with eager delight, and is the first person to laugh when he gets through. And sometimes, if he has had good success, he is so glad and happy that he will repeat the “nub” of it and glance around from face to face, collecting applause, and then repeat it again. It is a pathetic thing to see.”