This happened to my little sister, but it's still hilarious.
When she was in third grade everyone in the class was sharing their genealogy. Kids around the room were saying, "I'm English", "I'm Irish", "I'm Danish". My sister raised her hand and shouted out to the class, "I'm Germish!".
I was told it took a long time for the laughing to die down.
A childhood belief about nationalities

others believed things like this
- I'm autistic, and I used to believe taking some of my sister's food while she was eating it was sharing.Anon
- I used to believe that the Chinese boy in my kindergarten class flew from China every day to get to class.— Debi
- The boy in my class named "Sahand" must have a sister named "Safoot".Anon
- We told our little sister that she had a long bushy tail and she fully believed us for ages!!Anon
- I was told that spaghetti grew on trees and i anounced it in class one day in grammer school.— w.b.
- I remember a girl in my kindergarten class told me she wanted to be a waterfall when she grew up.Anon