I don't know if it is particularly funny, but when i was about 5 i thought that cats and dogs where the same animals, that dogs where the boys and cats where the girls. I based this assumption on my sisters nickname (kitty) and the fact that we would fight non-stop from morning to night (and i was a boy)
A childhood belief about cats and dogs

you weren't the only one
215 of you believed this
- I used to believe that cats were all female and dogs were all male.— Rachel
- that cats were only girls and dogs were only boys. I definetly didn't get the whole reproducing thing yet.— Jamie
- i used to believe a dog was the male version of a cat.Anon
- I believed that all cats were female and all dogs were male. — Biologically confused
- i thought cats were girls and dogs were boys. Anon
- i used to believe that all cats were female and dogs were male!Anon