I thought you only grew taller and bigger on your birthday. I would go and stand up against the wall ruler several times during the day of my birthday and check how much I had grown in the last hour. Figuring out that I grew all year long was harder to process than finding out Santa wasn't real.
A childhood belief about birthdays

others believed things like this
- iwas told that if u cry on your birthday you would not get any bigger 4 a whole year— steve
- I used to think you only grew on your birthday.Anon
- I used to believe that people only grew, physically and in age, on their birthday. — Aretia
- My parents have the same birthday -- month, day, and year. I though all mom's and dad's had the same birthday.— Cait
- 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 think that a birthday suit was a special outfit you wear on your birthday— Bri