At some moment, I thought cats were the females of the dogs, but I got past that quickly. However, it was much later that I've learned that the cows aren't female horses.
I imagined a kinda "filtering" system existed inside our bodies, to separate ingested solids and fluids, explaining that you pee when you drink and poo when you eat. :confused; I even drew some sketches of my theory.
I thought that by the time I'm 25 (which I am now), I'd have a family, a house, and a job, etc. Turns out, I'm single, just got my diploma and lives in an apartment. Add some debts to this too. :frown;
I thought you'd have to vomit at least once a year to be healthy. A kind of Spring cleaning, if you will. Not that I forced myself to puke though.
I was told AIDS was a disease that only men that "slept" together could have. I was quite shocked because I was sleeping with my dad (well, sharing the same bed only

) when I was visiting him on weekends.
I always had a wild imagination, I think there's a lot more I could say... But that's about what comes to my mind right now.