I thought that if you put peanuts in the blender, you would get peanut blender. So, I learned two things, one, that that is, in fact, untrue, and two, that peanuts will break the blender.
Also I thought that if you stirred milk long enough and fast enough that your would eventually get butter. I got bored after ten minutes of trying and decided that I wasn't "cut out" for the dairy world.
A childhood belief about general

others believed things like this
- I used to believe "mixed breeds" of dogs were made by two dogs being put in a blender— Andie
- i thought chickens laid peanuts.
— pikl - My daughter thought that ear wax was peanut butter.— Bobbiemac
- When I was a little boy, I thought that hair was made out of peanut butter.Anon
- I thought that barf was what they used to make peanut butter and jelly sandwiches!— Maan o' War
- I used to believe that peanuts were a daily must to strengthen the memory like elephants — Qb Cheese