When I was very little I would occasionally shower with my mum. I figured out quickly that she didn't have a penis, but couldn't quite figure out what was in its place. Her labia didn't close the whole way, however, and I was absolutely convinced that what I was seeing between them was her poop as it made its way to her bottom.
A childhood belief about rude bits

others believed things like this
- I used to believe that girls pee was in their poop, because they have no penis.— s
- I used to believe that overweight people were heavy because they didn't poop.Anon
- I used to believe that if you didn't close the toilet lid after flushing, you would get sucked down into it.— Maya
- I used to think that boys didn't get their penis until they were 18 which was legal adulthood! — beckles
- I used to think Hannah Barbera was one woman...not two men. I didn't figure it out until I was about 15!— minkie
- I always believed that we lived in Manchester (we didn't) but it was always the main place on the local news!— Richard M