This would be for those over the age of 18.
What is a very obvious and common knowledge thing you did not realize you were wrong about until you were an adult?
Very real example:
I can not tell what direction sound comes from, so I thought the whole “follow the sound of my voice” or “I hear water”, (then going towards the sound to find it) were just made up plot devices for movies and tv
I legitimately thought narwhals were fictitious until my 30s.
I also thought that when people got married on TV or in a movie, they were actually married. (We heard them take vows!) I thought this was why there was so much divorce among celebrities, because they had to get a divorce after filming.
I’m not sure I had ever heard of one till I saw the movie “Elf”, and was thoroughly confused as to what the heck it was or why it was there.
I didn’t understand why the “bridge freezes before road” signs were left up during the summer 
I only found out recently that travel pillows or U shaped neck pillows are supposed to be used in the front not the back
I was an adult before I realized that “x” was often shorthand for “cross” I was always ready for the “railroad xing” or “bike xing” to happen like a hotwheels set with the trains barely missing each other while making an “x”
I dated a guy that confessed that he thought the news was always reporting (events) at “Point Blank Ranch” and couldn’t figure out why people would even go there anymore.
I didn’t hear of them until I was an adult either (not from elf though I’ve never seen it)
I also didn’t hear of axlotyl until adulthood
I’m not sure they were originally intended that way, I think it’s one of those “someone found it works” and started telling everyone they’d been doing it wrong 
I used to think turning the oven light on would make the oven heat up faster.
I was in my 40’s before I found out that i had been flossing incorrectly my whole life. I was always yelled at for “not flossing” and eventually got to the point that I flossed 5 times a day and still got yelled at for it. Not once did any of the hygienists ever ask HOW I was flossing, they just assumed I was lying.
I do not think there is a wrong way really, unless you wear it as a hat hahah. I think it is own personal preference. But apparently it causes less neck strain in the front than the back 
I used to think the fridge light never went off and wondered how it lasted so long
me too!
I didn’t hear of them till just a few years ago when i saw a documentary on them.
I mean, I could see the logic…
I blame easy bake oven, that is how they work
True but I remember the packaging and print ads had it behind the neck not in front… which is why I think it was originally intended for behind
Yes that’s true
I was an adult before I realized that not every TV show theme song is sung by the actors in the show