When I was little, I thought that a boy's penis filled up with urine. I didn't know about bladders. Later on, I learned about testicles, and I thought that was what filled up with urine.
A childhood belief about rude bits

others believed things like this
- I used to believe that as I ate food, I filled up, and that once I filled up completely, I would die. Anon
- When I was little I thought that my urine was stored in my scrotum until I needed to wee.— Bryan
- i knew that if you ate beets your urine would turn pink and i thought if it was pink, then you could drink it.— JP
- I used to think that tampons were meant to absorb women's urine so they wouldn't have to go to the bathroom.— Koyaanisqatsi
- I used to believe that my urine was a super fertilizer - peeing on plants made them grow.— Andrew C
- I used to believe that monsters lived in my toilet and they ate my poop and urine when i flushed the toilet.Anon