How Fast Does A Baby’s Hair Grow, Anyway?

I am out with my two daughters, who are four years and ten months. We are at the checkout chatting with the cashier. 

Cashier: “What sweet children. And how fun! A little girl and a boy.”

Me: “Oh, actually two girls!”

The cashier points at my ten-month-old.

Cashier: “Oh, but she doesn’t have long hair.”