When I asked where babies come from, my mother explained that chemicals are exchanged from the man, into the woman, who then grows the baby in her womb. (My mom was pretty honest) I filled in the blanks though, the only real interaction I could think of to exchange chemicals was making out.
I used to believe that men had something in the back of their mouths, that would shoot out into the woman's mouth (ala Alien), and drop the chemicals down the throat into the woman's womb.
A childhood belief about reproduction

others believed things like this
- I used to think you had to be an adult to drink household chemicalsAnon
- 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
- My sister just recently realized that the lyric "more than a woman" wasn't 4 letter woman!— Anne
- I thought the line in the song "more than a woman" was "four-legged woman".— boo
- 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