When I first saw the movie 'Cats and Dogs'... Well, you can probably tell what my belief was. If you've never seen the movie, then I believed that cats and dogs had secret technology hidden right under our noses.
I did crazy stuff on behaf of my belief too. I checked my cat's litterbox for some sort of futuristic devices. No devices, but lots of poop.
A childhood belief about cats and dogs

others believed things like this
- Looking back at it, the belief that dogs eat cats is a pretty funny idea. I used to believe it myself.Anon
- 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
- Thanks to the movie "Annie", I thought hot dogs were made from real dogs. — Katie
- I believed that all cats were female and all dogs were male. — Biologically confused