Why is farting and fart jokes so funny?

What’s so appealing about seeing, hearing, and smelling gross stuff?

I can name three movies off the top of my head that contain some sort of bathroom gag.

American Pie where that guy has bad diarrhea
Harold and Kumar Go To White Castle where the twin girls are going poo at the same time.
Tenacious D Jack Black takes a dump with the weird guy stalking him.