When I was 3 or 4 I learned about menstruation from an exhibit at the science museum. It went into plenty of detail about how the womb's lining drips out the vagina for a week, but it didn't mention that there are little wearable products specially made to catch the drip. So for the next few years I believed that a period was a once-in-a-lifetime event where a woman has to spend the entire week on the toilet.
A childhood belief about periods

others believed things like this
- Up until I was ten I believed that Yesterday was a day of the week— Kirsty
- My mom told my sister that if she didn't eat her peas, her period would last for an entire month.— Maya
- My neighbors used to believe you could never count to 100 in a day, it would atleast take a week.
— raceystacyUS - I thought that "tomorrow" was one of the days of the week— Frank
- I used to want to be a garbage man when I was older because I belived they only worked one day a week— Oscar the Grouch
- I once refused to believe in Thursday. I'd heard there were seven days in a week, but could only name six.Anon