When my two brothers and I were little kids, we actually used to believe that there was an opposite day each year. We'd go around speaking to each other and everybody else in opposite meanings, like yes for no, etc. No one else understood what we were doing, and when we explained, "It's OPPOSITE DAY, DUH! You gotta speak opposite!" the adults would just look at us funny... The other kids, however, all immediately began to speak in opposite as well.
A childhood belief about general

others believed things like this
- I used to believe that the opposite of “impressed” is “pressed.”Anon
- I used to believe that if I could kiss my elbow that I would turn into a girl (the opposite sex).— Rusty
- i used to believe that everyone was born as the opposite sex, and that you changed when you started school.— llama
- I used to believe when adults grew up only 4' 9" or lower are still classified as kids.— Jorge
- I used to believe that school was a place kids go to get turned into adults.— Samantha
- I thought that kids' pee was yellow and adults' pee was clear.— Aretia