I consider myself pretty well educated on the subject of periods, and the basics of sex. But up until I was 16 or 17, I believed that both the pee and the period came out the same 'hole' (that is, the area where your pee comes out)!
It wasn't until I chanced a look down there that I discovered the vulva 'lips', and freaked out! Needless to say my Mum thought it was very funny when I told her worriedly, and set me straight as to where the period actually left my body (and of course where the penis actually goes during sex)!
A childhood belief about rude bits

others believed things like this
- I used to believe that if you left crumbs out for a long period of time, they would turn into ants.— Bylaberrie
- I was horrified when my first period arrived. It wasn't blue, like in the commercials for sanitary towels.— Jane
- My mom told my sister that if she didn't eat her peas, her period would last for an entire month.— Maya
- In 6th grade, I thought that once you got your period, you'd have it forever!!!! 24/7Anon
- I thought that your period came out all at once and you had to wear a pad around the time it would come.— Sarah
- i used to thought that period is when you pee like a baby for a few days...— i'm totally weird