My mother made me watch a children's program about childbirth when I was seven. It featured a boy whose mother was pregnant, and explained that there was a baby inside mommy's tummy and it would at some point come out and he would have a baby brother or sister. At one point, the mother's doctor shows the boy a little cartoon to explain how the baby got INSIDE mommy in the first place -- it explained that there were egg cells inside the mommy, and sperm cells that came from the daddy, and when those two cells got together, it started a baby growing. The boy is satisfied with this, and leaves.
Well, an hour after the program, I suddenly realized they hadn't explained HOW the cells got FROM the daddy INTO the mommy. I thought about it for a while instead of asking my parents and came up with an idea that made sense to me -- I assumed that the daddy wore a set of mommy's underwear for a day, so the sperm cells could "fall out" into them, and then mommy put them back on and daddy's sperm cells would then "go in."
Yes, for an entire year, I honestly believed all hopeful fathers were transvestites.
A childhood belief about getting pregnant

others believed things like this
- I used to believe that Mother Nature was The wife of God. She was the mommy and he was the daddy. — Dawn
- When I was a boy I flushed my fish down the toilet thinking that it would go to its mommy and daddy.— Jeremy Clark
- My brother used to tell me if I picked my nose I would become more stupid because bogey were brain cellsAnon
- I told my son that he used to be in my tummy when he was 4. So he asked me," Mommy why did you swollow me?"
— Christopher's mom - When I was young, I said to my mom, "Mommy, make the sun go down!"Anon
- Don't Laugh Guys But I Used To Believe Mommy Layed An Egg And Then I Was Born— KellGirl