When I was a child, I used to believe that the idiom "raining cats and dogs" meant that there were really animals falling from the sky, so I didn't understand when my parents told me that having a dog was too expensive !
A childhood belief about cats and dogs

others believed things like this
- My sister and I thought cats and dogs were the same animal, the dogs were male and the cats were female.— Paul
- I used to believe all dogs were male and all cats were female. I still call dogs "boy" and cats "girl." — Kelly F.
- I use to believe that cats and dogs were the same species; cats were girls and dogs were boys.— poopcrystals
- 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 that dogs ate cats.— Tom
- I used to believe dogs ate cats— LEADY