One time as a child, I was fishing with my Dad, and my hook snagged a balloon that was in the water.
For years I really thought it was a balloon that I had caught. Until I was about 16 or so when we were reminiscing…
My Dad just about laughed his ass off over that.
I thought that when a girl had to pee in a cup (like for the doctor) she basically had to float it in the water and hope for the best. Never occurred to me that she could, you know, hold it up against her body. :o
When I was very young, I had a habit of talking to myself without realizing I was doing it. I also had an EEVILL sister who was two years older than me. She used the stuff I muttered outloud (unaware I was doing it) to prove to me she was ‘magic’, and then would use that to make me do all kinds of stuff I wouldn’t have done otherwise. She would tell me that if I didn’t do what she told me to, she’d make me wake up at the bottom of the fish tank! I think I believed that until I was about 6YO. :o
That same sister thought that ‘satellites’ were something that cowboys put on their horses so they could see at night. 
My middle daughter (now 20), believed that ‘Pay-Per-View’ meant you tuned in to watch paper. 
I used to think that fog was sleeping clouds.
My brother had a little tank with 3 goldfish, and one day they were all dead, floating at the top. That evening my mother served custard for dessert; For many years I wouldn’t eat custard, being convinced that it was made from dead goldfish.
I thought that “vanilla” ice cream had no flavor at all, that everything was vanilla before they added flavor.
I thought that all black men had moustaches.
I thought the loser in a presidential election became vice-president. And of course the cabinet secretaries were actual secretaries.
Occasionally, I would hear somebody mention the dumbwaiter in the building, the dumbwaiter near the hall, the dumbwaiter in the corner… I thought it was mean for people to refer to their co-worker as “a dumb waiter” and secretly wanted to meet him to apologize for his co-workers.
I’m embarrased to admit I was at least 12 before somebody corrected me.
My dad used to take me on walks around a local park that had a nature trail. He would tell me that the pond scum was “snog frott,” which was one of his made-up hillbilly words for any disgusting natural substance (pond scum, dog slobber, etc). On a class field trip to a nature center, I pointed out some pond scum to our guide and insisted to her that it was snog frott, that my dad had told me so. She kinda went “okaaaaaaay” and moved on. My dad laughed his ass off and left me to figure out what a jackass he was years later.
I was told that if I picked my nose and ate the results, I would get worms. I would also get worms for chewing my fingernails.
I believed that the people of Wisconsin all dressed like Heidi or Swiss Miss. Evidently the fact that many Swiss immigrants settled in western Wisconsin and seeing the movie “Heidi” on TV as a child got mixed up in my brain. I was soooo disappointed the first time I went to Wisconsin to find out otherwise. I was 8 y.o. at the time.
So, was it a condom?
Edit: To add my own; Speaking of condoms, I thought they were invented in the 1980’s because that’s when I first heard about them.
I thought all music was recorded live, and used to wonder what ending the audience heard on recordings with a fadeout at the end.
When I was very young, probably 2 or 3, I went on a field trip to an important government building in DC, possibly the Capitol, and for some reason believed that the man in the front office who signed us in was the current US president, George Washington.
I convinced myself there was an alien hiding behind the mirror on the wall in the dining room. Because he was an alien, he could make himself perfectly flat using his alien powers. He had light switches back there for all the lights in the house (I was picturing something like the breaker box) and every time the lights flickered, it was the alien turning the lights on and off.
Once my mother told me that when you get a black eye, “your eye turns black, and then purple, and then green, and then yellow, and then it goes away.” I interpreted this quite literally and was briefly afraid that a black eye would eventually cause your eye to disappear. I later got a nice shiner when a seatbelt buckle hit me in the face, and I was quickly corrected on that one.
Another misinterpretation: someone once explained surrogacy to me, and what I took from that was that sometimes a baby would disappear from one woman’s uterus and randomly end up in another’s. I thought this was really exciting for a while and tried to come up with a story about a woman who randomly gets pregnant by this method. A bit more thought made me realize how patently ridiculous it was.
When I was a kid, I lived on a circle street with a streetlight in the middle. I used to think that one of my neighbours had the switch in their house that turned it on and off.
I thought that when colour TV was introduced, so was colour itself. Prior to that, the world had been black and white.
I would call BS and say you stole that from a Calvin and Hobbes strip, but I used to think the same thing. I also used to think the Old West used to be sepia-toned.
I thought that since you could make a color TV go black and white by turning a knob, you could do the opposite with a black and white TV. My friends had color TVs and we only had a black and white one, and I got soooo frustrated trying to make it happen that I ended up scribbling on the screen with magic markers while the program was on, attempting to colorize it.
Also I found that if I pressed the sides of my eyeballs gently, I could get mild double vision. I thought that this was a way to make the TV go 3D.
At the age of 12 my sister asked me if we flew straight up from the UK whether we’d arrive in America. Turns out she knew the earth was a sphere, but thought we were on the inside if it.
At school we were always told: “Never cross the street near a parked car.” But why?! Nobody ever told me why. I honestly thought it was something to do with etiquette, until I was old enough to ride in the front seat and observe a driver’s PoV.
My biggest thing I REALLY believed what the elusive jackalope.
When I was 7 years old we took a trip out West, and there I first saw it. Oh it was on postcards, in travel books, it was on gift stands, the jackalope.
And it was my duty to find him. So for two weeks from Minnesota to Yellowstone and all the way back through South Dakota and Nebrsaka, I saw him in photos everywhere, but how come I couldn’t see one for real. I saw hawks, owls, prairie dogs, buffalo (bison), everything else.
It wasn’t till two weeks after I got home that my brother broke down and told me, “Mark it isn’t real.” I was crushed.
What really amazes me is the huge number of adults I asked during our trip about the jackalope and you know, not ONE ADULT let on that it was fake. They all encouraged me to go find it. 
I used to live in a small country village, near a church which rang the hours all through the night. I was really torn between thinking that ringing the bell was a great job- after all, he must only work for like 24 minutes a day, or a rubbish job, because you wouldn’t be able to spend very long away, and if you were in the middle of dinner out or something you’d still have to run back and chime it and your friends might get annoyed. I used to look out for someone running towards the church just before the hour, but never saw the same person running that way twice, so I wasn’t sure who it was. I thought probably there were two people who split the job so they could go shopping and on holidays and things.
I was very surprised when I went with the girl guides into the tower, and I couldn’t see a bed anywhere- but then they let us try and ring a bell, and I thought no actually, that made sense; he’d get a headache if he actually slept in the room, because they echoed for quite a while. I didn’t ask where he slept though, I thought that would be rude, and the person giving the tour clearly wasn’t the person who rang the bells for the hours, they only did weddings and special occasion stuff.
I remember the one night when I was fuzzily listening to the clock chime 4am, and suddenly thought, ‘He must be really tired, why don’t they just get clockwork?’ then it hit me.
I’m still a bit disappointed.
I believed that human history was a steady march of progress and improvement and that we had achieved the very best state of being that could possibly be achieved.
I believed that government officials in Washington were chosen for their wisdom.
I believed that all decisions about society were made after careful consideration of the facts, and if the decisions were wrong it was because no one knew the right answer.
What a chump I was!
Isn’t this the essence of the philosophy that Voltaire satirized? The “this is the best of all possible worlds” idea? (I forget what it’s called.)
I thought it was rude to refer to a person who practices Judaism as a “Jew.” I thought the more polite term was “Jewish person,” and that saying “she’s a Jew” was like saying “She’s a n*gger.”
Yes indeed. Oops, I left out that little bit of information. :smack: