As a child, once I got clear on the fact that I do have an anus (rather that poop just coming out by a sort of osmosis from that general region), I thought that an anus opened up into a cavity large enough to mostly fill one's entire abdomen. Turds would gather in that large cavity, so I thought, and compete to be the first one to get out. I thought at the time that a fart was a case of a turd trying to get out but missing the hole, perhaps being pushed aside by another turd.
A childhood belief about weeing and pooing

others believed things like this
- I used to think that if you held a turd in for more than an hour it would give you bad breath— phil
- I belived that when I took a crap, my turd(s) would turn into snakes and bite my booty.— Butters.
- I used to believe that very large rocks and boulders were actually petrified dinosauer poop.— Maureen
- I thought a major general is more powerful than just a general (which still kind of makes sense to me)— Demond
- I used to believe the large metal structure in Paris, France, I thought it was the "Eye Full Tower"— Janet
- At one time I believed that ZZ Top was an unusually large bustline.— Donna K.