My brother was the only boy with 4 sisters, and somehow got the idea that girls all get their period at the same time, and it lasts for a month (the term monthly?). He was in high school before a friend set him straight.
A childhood belief about periods

others believed things like this
- I used to think that girls' anuses were green. I remember thinking that as late as high school.— Jack
- My mom told my sister that if she didn't eat her peas, her period would last for an entire month.— Maya
- I used to think making straight A's in school was literally writing A's straight. Anon
- For years I thought that spunk was a male period and that boys spunked once a month - Yeah right!— Stupid Child
- That you had to marry your sibling. During this period of time I devoliped a crush on my brother.— don't worry I'm a girl
- that the money I gave at sunday school would go straight to God.— christine