I always thought that carbonated beverages like Pepsi, Coke, Sprite and suchlike were called fountain drinks because they often "fountained" when you opened the can or bottle. It wasn't until I was 20 and had to maintain a drink fountain as part of my job that I realized the machine that dispensed the drinks was called a fountain. That's where the name comes from, but I still like my idea better.
A childhood belief about drinks

others believed things like this
- when I was little, I called coke and pepsi "Black Juice"— can I have a refil on the black juice?
- i thought that soft drinks in America would explode in a person's mouth bocause they call it 'pop'.— Aussie!
- I thought that soda was also called "pop" because if you drink too much of it, you'd explodeAnon
- I believed that the breed of dog called Boxer was called that because they could stand up and box.Anon
- I used to believe that, no man drank hot beverages, because my father never did. — Sue
- I used to believe that people from Holland were called "Hollish", like people from Poland are called "Polish"— Sally