brain + anal = brainal
This situation occurs because the brain in your anus overrides the brain in your head. Despite your best efforts to clinch and tighten, your brainal can sense when a toilet is near and immediately starts to open the brown gates.
"the ass always knows"
Jimmy Neutron: Mind-Fuck!!!!!!!
Jimmy Neutron: BRAINAL!!!!!!
*head explodes and sprays copious amounts of shit and jizz across the walls, and a little trickle lands on Nietzsche's Uber-Stache...
*Tongues his 'stache suggestively.
Nietzsche: Bon appetit