When I was small I had a toy measuring tape that marked every centimeter (about 0.4 inches). I came to the conclusion that a centimeter is the smallest unit of length. So when something was, let's say, 0.5 centimeters long, I though it's either a measurement error or a pure illusion.
A childhood belief about physics

others believed things like this
- I used to believe that ants had the smallest nipples in the worldAnon
- I believed that linguini was made of tape worms and was scared to eat pasta for a long time!— Mojo!
- I used to believe that Africa was the biggest country and Luxembourg was the smallest.Anon
- I thought the “rec” button on tape recorders said wreck and it would wreck your tape if you pushed it!— Alberto Richardson
- When I was little I used to believe that if you told a lie, your nose would grow in length. — Lluna
- I used to think that if you went to the end of a road marked "Dead End", somebody would try to kill you.— Spooky