The sad thing about this one is that I wasn't even a kid - I was a young teen with new ZITS! I worked out in my head the idea that I could carefully but effectively remove them with sand paper. It did remove them in a sense, but did not have the desired effect. And it really hurt! A stupid believe (theory) but a true story. (PS - I only tried it once.)
A childhood belief about skin

others believed things like this
- I used to believe that zits were chicken pox.Anon
- I could NOT understand why, if a pen worked on paper, it couldn't also write in thin air.— Can you guess what it is yet?
- I used to think that when a movie was 'based on a true story' it was happening for real then and there— Tom
- When I was young I had a plan to shoot big piles of paper up to the sun so it would never burn out.— Waterbottle
- I used to think that if you ate cat food you would die. I found out it wasn't true
— Bob#4 - I believed when young that horses with white faces were sick. My dad a great sense of humour— John Cavanagh