I used to believe the opposite of "expensive" was "relading". I didn't even know what expensive meant. I was just told I couldn't have something because it was expensive, so I said, "No! It's relading!", not knowing what the word meant but wanting that.
A childhood belief about money

others believed things like this
- I used to believe that good and expensive joggers would make me run fast.— Kats
- I thought braces were made out of precious metals and that's why they were so expensive!— Sp' Iyulgar
- I used to think that an imbecile was some type of seal, before knowing that it meant someone stupid.Anon
- my husband believed, until recently, that the word "several" meant seven of something. — sheryl
- I used to believe that "nuptial" was a naughty word. I thought it meant nipple. XD— Sara Hagen
- i used to think the word "friendship" meant a special kind of rocketship that you went on with your friends.— chips ahoy!