The other day, I was failing to teach my 3-year-old son about measurement. He wanted to figure out if something would fit in an envelope, and I was “helping” by showing him how to measure the width of the envelope, then comparing it to the width of the paper he was trying to insert. It turns out that this is a trickier concept than I had assumed; the ability to understand even simple measurements requires a fair amount of cognitive maturity. 3-year-old kids can compare directly, but the c...
