I used to believe that numbers stopped at 99. To prove me wrong, I distinctly remember my older sister using our basement chalkboard to write every single number from 0 to 99, and then writing 100. This may or may not have convinced me at the time, but eventually I did learn the truth.
A childhood belief about classes

others believed things like this
- I believed that numbers eventually just ended. That they stopped somewhere after 1000.— Jamis
- I used to believe that if you had an animal listen to classical music, it'd eventually learn how to talk.Anon
- I used to believe that numbers had gender, that the odd numbers were boys and even numbers were girls. — Evelyn
- My older sister had me convinced that noodles grew on trees, but that rice was man-made...— Trusting little brother
- When I was little - before we learned cursive - my sister and I called it 'real writing'. — Monty
- I used to believe that after the number "ten" came the numbers "tendy-one, tendy-two..." etc.Anon