(I can't find an appropraite category for this one...)
When I was younger, my family was in the military and moved around between countries. Thus, we always had passports.
It wasn't until high school that I learned that passports are not required by citizens until they leave the country. I'd always thought everyone had a passport from the day they were born.
A childhood belief about nationalities

others believed things like this
- I used to believe that the moon was another country and that all countries were little planets.Anon
- Since I was born on the 4th of July, I really thought the entire country celebrated my birthday.— -Stoddard
- when i was younger i thought that when you moved house your house moved with you!!— chrissy
- When I was younger I thought that the jets that leave lines in the sky were rockets — Harry
- My younger sibling thought that Brittany was the name of the country where British people livedAnon
- I thought that wars had to last four years... just like high school and college.— A. Falk