Oh, to be muppet Moe Sizlack!
Of course, The Simpsons were playing off of the old "That's not my belly button" joke that I heard growing up. There are many versions out it out there; here is one (this one from aardvarkarchie.com
"Class dismissed!" the teacher yelled but little Johnny doesn't go.
He walks to the teachers desk and says, "Teacher can i go home with you?"
The teacher says "No!"
Little Johnny says, "I'll tell my daddy."
So the teacher says, "Okay."
They get to the teachers house and she says, "Well i'm going to take a quick shower, you sit right here."
"Can i take a shower with you?" he asks.
The teacher says "No!"
Little Johnny says, "I'll tell my daddy."
So the teacher says, "Well okay, I guess."
So their in the shower and little Johnny says, "Can i turn off the lights?"
The teacher says "No!"
Little Johnny says, "I'll tell my daddy."
So the teacher says, "Okay."
So the lights are off and little Johnny says, "Can i stick my finger in your belly button?"
The teacher says "No!"
Little Johnny says, "I'll tell my daddy."
"Well okay." says the teacher, "JOHNNY!, that's not my belly-button!"
"Yeah? and that's not my finger eather!"
I've also heard a version with a couple the night before their wedding, etc.
