The recent Veterans Day reminded me that I used to think the Tomb of the Unknown Soldier had the body of the one guy, out of all the millions killed, that they couldn’t identify.
I thought that gas stations were built above underground deposits of petroleum, which was why they were clustered together on street corners. How they usually managed to be at the corners, I never considered.
That’s a pretty reasonable assumption for a kid to make. You were still a kid, right? 
Yes ,very young. So young that I misheard PSAs about tuberculosis. They always called it “TB” and I thought it odd that they railed about TV on TV.
I thought the same thing. We all believed that to be common knowledge.
I used to think that you needed to say “Excuse me” after you farted because that would make the smell go away.
I swear my mom told me that. She swears she did no such thing.
My mom did tell me not to lick butter off the knife because I would cut my tongue. For years, I thought it was eating butter by itself (e.g., no bread) that would slice your tongue. (Rather than the knife…)
Yeah, I have a PhD now…
I thought that you could plug a power bar into itself and then you’d get power to the other things you’d plug into the power bar.
When they taught us the Lord’s Prayer, they didn’t bother to explain that “The Lord is my shepherd, I shall not want” didn’t mean you didn’t want Him.
The term guerilla warfare was thrown around frequently during coverage of the Vietnam War. I kept expecting to see film of gorillas shooting at each other. Took me a while to figure that out.
When I was a kid (early 1960s), I saw public service ads reminding all aliens to register at the Post Office. I didn’t know this applied to those of us born on this world and had visions of Superman walking into the Metropolis P. O. to register his status.
When I was quite young, I became aware that matter was made of atoms, and every atom has a nucleus. And I became aware that living things are made of cells, and every cell has a nucleus.
Thus, I concluded that atomic matter (including compounds) and living matter are two mutually exclusive kinds of stuff. Cells are the fundamental unit of living things, not composed of atomic matter.
And thus began your career as a theoretical physicist, right?
Here’s a complicated and very specific one. One day my School Teacher told us the story of Jason, the Argonauts, and his search for the Golden Fleece. However, the local pub that served our community was also named “The Golden Fleece” (it was a farming community, so it was both a reference to that and the myth, but I didn’t know that) and at no point did the Teacher make it clear what he meant by a Golden Fleece, so all I could picture was the pub and not a sheepskin made of gold wool.
It was at least a decade later when I saw the story retold that all of that finally made sense to me.
The first time I saw someone speaking a language other than English (I forget which) on a telephone, I wondered how they’d trained the phone system to be able to cope with it.
Reminds me- not that many years ago my boss was musing about GPSs in cars, and was amazed that the satellites could keep track of all those cars. He wasn’t physically young.
In a similar vein, the line in “Silent Night”
'Round young virgin, mother and child
Well, of course she’s round – she’s pregnant.
Seems like birds used to swoop down on cars more back then. I used to think that it was because, with their peculiar eyes, large things moving at a distance look like little things close up. I finally learned that for a zillion years large animals moving through the grass stirred up bugs for them to eat.
When I was quite young my parents taught me that a woman got pregnant when the father’s sperm combined with the mother’s egg. But they didn’t teach me the part about how the sperm got from the man to the woman. So I concluded that sperm floated through the air, more or less like pollen. So when you got married living in the same house and sleeping in the same bed with your wife increased the probability of one of your sperm landing on her, which is why you had kids after you got married. And out of wedlock pregnancies must happen when one of your sperm accidentally lands on the wrong woman.
Nitpick: That line is from the 23rd Psalm, not the Lord’s Prayer, which is the “Our Father who art in Heaven” one.
I thought it was amazingly fortuitous that the English translations of words that rhymed in foreign-language poems just happened to rhyme in English too. Because look, here’s the rhyming poem in its original foreign, and next to it is the translation that rhymes in English. Cool!
At the time I hadn’t quite got my head around how the translation process actually works.