The rim of your anus typically with a circumference 2πr, r being the radius of the ass pussy. This has an exponential growth whether you are excreting waste, or receiving sweet love which is proportionate to each persons bum cheek volume.
Jonathan dropped the soap and felt the exponential growth of his bum lips
Ruler of Lake Titicaca. Rumored to have a bunghole that gets very angry if it does not receive toilet paper. Cornholio the Great is often seen walking around with his shirt over his head and his hands in the air, chanting songs about his power, and his bunghole.
"I am Cornholio! You do not want to face the wrath of my bunghole, for I need TP!"
Butthead: Shut up, Beavis! (uh huh huh huh)
Beavis: Um, okay. (heh heh heh heh).