Suppose to be kid friendly so don't expect them to ever give you an answer in the comic.
Just ask yourself these questions.
Does he break the toilet everytime he takes a dump?
Is there a shockwave if he farts?
Did Ma Kent have to keep replacing her toilet because Clark kept blasting a hole through it when he peed?
I'd say no. Superman's bodily functions seem as normal as any other humans.
Haha, I sigh everytime this question comes up because everyone who asks this thinks they're being really original and funny. I laughed my butt off when I first heard it in Mallrats, but now it just pops up every once in awhile.
All I can say is that yours is the best answer I've seen so far. I'd always just say he's Superman, he has super-control of his body
. But your answer is priceless.