When I was younger I used to think that a period was when you poop blood cells out of your body and I always referred to it as "blood cells." When I was about 6 or 7 I learned it was called a period.
A childhood belief about periods

others believed things like this
- HA! I used to think when you got your period, you would just pee blood...I wish it was like that...if only.Anon
- I used to believe period blood came from the nipples. I thought that was what "padded bras" were for!!Anon
- My brother used to tell me if I picked my nose I would become more stupid because bogey were brain cellsAnon
- when I was young, I heard that blood had iron in it. I thought blood was red because it was rusty.— Hunter
- I used to think that instead of bones and blood and tissue that you body was made up of tuna.— m.n
- I used to believe that guns shot out blood because everytime somebody shot a gun in a movie you saw blood.Anon