I used believe that when you had your period the blood would come out of your vagina, and your nipples. So i thought the purpose of the bra was to keep the blood from leeking out of that area.
A childhood belief about periods

others believed things like this
- I used to believe period blood came from the nipples. I thought that was what "padded bras" were for!!Anon
- I thought pads sucked blood from women. It wasn't until later in life that I knew their actual purposeAnon
- when I was young, I heard that blood had iron in it. I thought blood was red because it was rusty.— Hunter
- 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 that guns shot out blood because everytime somebody shot a gun in a movie you saw blood.Anon
- when i was litte
I used to believe that white people had white blood and black people had black blood.Anon