Alright, have two for you.
When I was around 6, I stumbled upon my mom's tampons. For some unknown reasons, I thought they were used for catching flies. So every day after school I'd unwrap a tampon, take it apart, and try to catch a fly in it. When I finally managed to, I ran down to show my mom. She got a good laugh out of that..
Secondly, around the same age I also found mom's pads. Being the curious kid that I was, I unwrapped a few of those two. I thought that they were a special kind of slippers. I stuck two to the bottoms of my feet, and walked around on them for a long time, before my mom finally noticed, and took them away. Bummer.
A childhood belief about periods

others believed things like this
- I thought that a woman got pregnant kind of like catching a cold, by being around her husband for a long time.— No Name
- I used to believe that some day, they'd finally let the Coyote catch the Road Runner. I was so wrong. Le Sigh.— Givema Break
- i used to belief that tampons were rolled up pads in a tubeAnon
- few years ago, finally I found out that asparagus are not big sea fishes.. — nixass
- I thought that gnats were baby flies, then one day I found out about maggots. yuck— Felicia
- My parents told me I was found under a gooseberry bush which I thought odd for a long long time.— Kate