I thought "birthday" was actually "Bert day" to celebrate Bert from Bert and Ernie. My mom must have thought I was just mispronouncing it because she didn't correct me until I was about 7 years old, at which point she had to explain to me what a birthday was.
A childhood belief about speaking

others believed things like this
- I seriously used to believe that Bert from Sesame Street lived in my closet and that he was going to kill me.— Liz
- i used to think that if i didn't celebrate my birthday each year my younger sister would catch up with me— q
- Up until I was almost 11, I thought that a birthday suit was a special outfit you wore only on your birthdayAnon
- I used to believe that on your birthday, you didn't advance to your next age until you ate your birthday cake.— Casi
- I used to think that a birthday suit was a special outfit you wear on your birthday— Bri
- i thought that when it was your birthday you grow and then you didn't grow until a years time.— hello