As a kid, I'd only ever heard the word "Gesundheit" used in cartoons, so I thought it was a meaningless funny-sounding word that was only ever used in cartoons. Well, one time when I was visiting my neighbor (who was German), I heard her say "Gesundheit" after I sneezed. From then on, I thought she must have liked watching cartoons a lot.
A childhood belief about foreign languages

others believed things like this
- In first grade, I thought that if I've never heard of a word before, then it wasn't a word.— Destiny
- I thought live-action films were very, very well-drawn cartoons.Anon
- I thought cartoons were for kids and "real-life programs" for adults.— Ichigo
- I thought cartoons were real, I remember asking my mom why there were lines around the "actors" in a cartoon.— Steven Graziano
- Because of cartoons, I thought that to drive you'd just turn the steering wheel back and forth constantly. — Mitsuki
- When I was young, I heard the word 'minion' for the first time and thought it was a miniature onion.— Karin