Young mathematicians fool us into thinking they know how to use a ruler. I've seen a study that proves this is true. Shown a ruler and object like the one below and most students, even into the upper elementary grades, will give a wrong answer. Their understanding of measuring units is faulty.
This is why a year ago I started attacking this misconception and posted about it here (freebie included) and here. This year we tried the same activities, but I decided to make a class book out of the concept. We need all the additional practice we can get!