I once convinced my little brother that everyone has purple hippopotamus that only they can see and that if you don't have one, then it means you aren't a real person. He cried for hours because he couldn't find his purple hippo.
A childhood belief about imaginary friends

others believed things like this
- My big brother told me that if you peed in the pool, it would turn purple and chase you.— Elizabeth
- When I was little, my dad told me that if you pee in the pool the water will turn purpleAnon
- I used to believe that God was the Incredible Hulk - he was green and muscly and had purple trousers.— Emma
- When I was younger, I used to think that barney was a giant purple and green puppy!Anon
- if you drank coffee before your 16th birthday, your skin will turn purple.Anon
- I used to believe that people's brains differed in colours. I always wanted purple brains for some reason.— Katrina