Sorry if there’s already a thread on this; I looked and didn’t see one.
Wrong things you thought when you were young?
In my favorite movie, Who Framed Roger Rabbit, Eddie Valiant (Bob Hoskins) stands outside his detective agency and is handed the mail by his postman. Eddie wants to know what he’s got, and the postman says, “The usual bills.”
Eddie throws the bills in the nearest trash can.
I was seven years old when I first saw this movie, and because of this, I thought bills were junk mail.
States have actual lines around them. I was very surprised the first time my folks took us out of state by car, how did anybody know where the edges were???
As a child I thought the world would make sense when I was older.
As a child, I thought people went to work because they wanted to work, and that the Government gave everyone money, The older you were the richer you were. It all made perfect sense to me at the time.
I thought if I prayed hard enough, God would let me live as long as Noah.
I grew up in Los Alamos. Both of my parents had PhD’s and most of my friend’s parents were scientists, so I grew up thinking that the standard educational tract for most people involved 12 years of public school, 4 years of college, and 7 years of grad school.
All dogs were male, all cats were female.
There were only 2 religions a person could be: Catholic or Jewish.
The world was rendered in black & white when my older relatives were younger.
I thought this too. I also thought being an adult would be fun. And easy.
How wrong I was.
Well, it’s working so far.
I grew up knowing I would one day go to college. What I did not know, and what troubled me deeply for many years, was that all students didn’t have to play on the college football team, as if it were a mandatory Phys. Ed. class which happened to be broadcast on TV.
I thought my grandparents had come directly from primordial goo, apparently I was unable to conceive of the depths of time between them being born and 1982 
I thought sex was something only boys/ men desired.
Oh, and I was very surprised the first time I heard a girl fart.
I thought everyone changed names once they reached age 10 or so. When I was six, as a result, I was flying on a plane one day and a flight attendant asked me “What’s your name?” I replied something to the effect of “I haven’t decided yet” (since I figured my name change was upcoming soon.)
I also thought everyone pays taxes on their entire personal wealth, such that a billionaire, if charged a 30% tax rate, would be totally out of his wealth in less than 4 years.
Oh, and I thought “go on a diet” meant “eat a lot of food.”
That would be a FANTASTIC reality TV show idea.
I thought Starsky and Hutch were the ultimate forbidden love couple. It was so touching and sad. I was sure they would get married if it was physically possible but, because of the laws of science and/or physics, it was impossible for two men or two women to marry.
At age 5 or so, I thought Lincoln had freed the sleighs which had gotten stuck in the snow. I suspected Santa may have been involved too (perhaps it was Santa who was stuck) but wasn’t sure.
I think I mentioned these in the older thread, but they are worth repeating.
At a young age my parents taught me that a woman got pregnant when the father’s sperm fertilized the mother’s egg. But they didn’t explain the part about how the sperm got from the man to the woman. So I concluded sperm floated through the air, like pollen. And based on that, out of wedlock pregnancies happened when one of your sperm accidentally landed on the wrong woman.
The old commercials for Dow Bathroom Cleaner showed a bunch of anthropomorphic cartoon soap bubbles emerging from the can and cleaning a bathroom. I believed that was what would literally happen if we bought some, so I begged my mom to buy a can. I was very disappointed by what actually came out.
I got Transylvania and Pennsylvania mixed up in my mind. In other words, I thought Pennsylvania was where Count Dracula was from. I was disappointed when we drove through the state and it wasn’t spooky like an old Dracula movie.
Growing up in the 1950s, I for a while thought all TV was real. After all, Lucy and Ricky (Desi) were real people, the Nelsons were a real family, Jack Benny was a real person, et cetera, and a lot of TV was “live,” so until I was maybe 6 or 7, I thought it was all real.
I also thought there was some rule that you had to have snow on Christmas, because the song said so. I remember my disappointment one Christmas morning when I found out Bing Crosby had been lying.
Even though the whole point of the Bing Crosby song was that it wasn’t snowing on Christmas. At least, not in Beverly Hills, LA.