A teacher I know told me this one: she had a female student who never heard that women could produce milk and nurse babies. When the student put two-and-two together and finally understood, she blurted out, dumbfounded "LIKE COWS???"
The kicker (and I'm not lying here) -- she teaches pre-med students in COLLEGE!
A childhood belief about reproduction

others believed things like this
- I used to believe I could milk dogs like the cows. This was because I saw a nurse milk his cow.— Laurent and Alexis, French students in eighth grade
- I used to believe chocolate milk came from brown cows and regular milk came from black and white cowsAnon
- i used to think chocolate milk came from brown cows and normal milk came from white cows Anon
- I thought that a "student visa" meant to use a credit card while travelling.Anon
- I used to believe that brown cows made chocolate milk and white cows made regular milk.— andrew mient
- I used to believe that brown cows gave chocolate milk, and that white cows gave white milk.— James