Robin Williams Favorite Joke

Guy's having sex with his wife. All of a sudden he looks up and there in the doorway is his 8-year-old son. Kid looks horrified and runs away. The guy says to his wife, "Well, I'd better go talk to Timmy." He puts on some clothes and goes to Timmy's room. He opens the door and there's Timmy banging the shit out of Grandma. The father says, "Oh my God!" Timmy says, "Not so funny when it's your mom, is it?"