I used to believe that rats and mice were the same species, rats being males and mice females, and that similar to the b-word meaning a female dog, the word "rat" was a swear word. Euphemistically, I referred to rats as big mice.
A childhood belief about swearing

others believed things like this
- I used to think thats rats put those eye crusties there while I were asleep— Maria
- I used to believe that tangled hair really had rats in it.— Kristy
- i used to think the word "suck" was a swear wordAnon
- I used to believe that spiders were mutated rats that were shrunk!— Quako
- I used to believe that meatballs were made from dead rats.— Dogface
- when i was small i thought rats mutated into batsAnon