I thought that when you peed, your pee would go to a place where it was manually boiled on a stove, making it 100% clean drinking water, and then when you turned on the tap, they would get a garden hose and pour water into the pipe that gave you water. Somehow I assumed this all would be done manually with normal sized stoves and kettles, and that each tap had it's very own pipe.
I also thought that black people were just tanned, because colder countries had white people and warmer ones had black or brown people. Then I asked myself: why are there black people in cold places? I came to the conclusion that all the black kids in my kindergarten would eventually lose their tan and become white.