21. Ralph Won't "Morph" If You Squeeze Him Hard Enough (Season 6, Episode 11 - Fear Of Flying)
There's nothing smart or clever about this one, but it is pretty damn funny. It's safe to say that considering the majority of the Simpsons' chalkboard gags are about comments Bart has made or actions he has taken, he must've tried to make poor Ralph Wiggum "morph" by squeezing him into oblivion. And of course, the thought of such an event occurring is much funnier than if it had actually been shown in the episode itself.