What childhood misconceptions did you have until you were older than you would care to admit?
Personally I thought "diet" was just a flavor of soda/pop like the cherry versions of coke/pepsi. I always preferred diet to regular as regular was too sweet, even for child me). Long after I learned what calories were(the chemical energy needed to heat a cubic centimeter of water one degree or something?) a friend took a friendly jab at me for drinking diet to watch my weight or something. I confusedly asked him the heck was he talking about and a quick look at the nutrition facts printed on the can revealed the zero calories compared to the regular hundred and something. I was 13.
A friend of mine thought only girls were supposed to use shampoo and had been using bar soap on his hair. He did not correct this mistake until High School, when there were group showers after gym and after school sports and the like, and a fellow athlete asked to borrow some shampoo.