When I was in 4th grade, I got to stay overnight at a classmate's house. Her big sister--who was in 6th grade and therefore a font of information--told us about where babies come from.
She said that the man and woman take off all their clothes, get into a bathtub together "and the woman rubs the man's 'thing' until it foams and foams. It fills the bathtub with foam and then they take the foam and rub it all over the woman's skin. And she becomes pregnant."
I actually believed this for a few years.
A childhood belief about reproduction

others believed things like this
- My sister just recently realized that the lyric "more than a woman" wasn't 4 letter woman!— Anne
- I believed that the yellowish foam by the ocean was dolphin pee because my dad told me Anon
- I thought the line in the song "more than a woman" was "four-legged woman".— boo
- I used to think that if a black woman or man and a white woman or man had a baby, it would be Asian.Anon
- I used to believe once a man and a woman lay naked caressing each other, the woman would be pregant— oiman
- I used to think that the song "Evil Woman" was "Medieval Woman". Anon