I didn't know the meaning behind the songs Master of Puppets (Metallica), Flying High Again(Ozzy), Crazy Babies (Ozzy), Sweet Leaf (Black Sabbath) and many other songs about drugs until relatively late. I found out about Master of Puppets being about drugs in middle school (2002 or so). I found out about Flying High Again being about drugs a few months ago (I'm nearly 25).
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When you die, if you've lived a long enough life, you're given a Tardis-like room which is infinitely large where you are able to create your own universe. Once made you are free to shrink down or scale up to interact with your own little creations.
I thought Dungeons and Dragons was something that people used to do before video games and, once Video Games became a big thing in homes, everybody stopped playing D&D forever. I didn't think I would ever play a game of D&D or anything similar and I'm glad I was wrong about that.
Cartoon and Comic characters existed. Furthermore I thought Clowns were their own sub-species of humanity.
Finding out that we as humans didn't have colonies on the Moon or Mars was probably worse for me than finding out Santa Clause didn't exist. Finding out Dragons didn't exist was also really jarring to my young world view.
I didn't know California and/or Orlando and/or New York City was where every TV show was filmed. It disappointed me that I would probably never be on any of the shows I loved as a kid like Legend of the Hidden Temple or Nick Video Arcade...