Many parents track the height of their child by having them stand next to a designated wall every year or so and making a hashmark just above their head.

I'd use this method, if I trusted myself to remember to do so every 12 months. Fortunately, I have figured out an alternative way to track The Squirrelly's growth. Whenever I have him on my shoulders, walk through a doorway, and hear a "Twump!" from above, I just stop for a moment and jot the current date on the wall next to the frame.