I believed that girls were born as boys, and boys were born as girls, and you changed when you were still a baby.
A childhood belief about reproduction

others believed things like this
- i used to think boys were born from men and girls were born from women— swag lad123
- When I was 5 I thought that baby boys came from men and baby girls came from women...— Gaz
- I thought that the difference between girls and boys was that girls have teeth.
Anon - I used to believe all babies came out as girls and later on in life some of those girls turned into boys — Tom
- I used to believe that men had baby boys and women had baby girls.— Janis Smith
- When I was a little girl, I believed that all cats were girls and all dogs were boys. :)— Mary Ellen