Use it or lose it.

I just hiked out of Bryce Canyon, emerging at the rim edge with about half an hour of daylight to spare. I feel very accomplished, as I climbed 580 vertical feet--up to an altitude of something like 8100 feet--in something of a hurry. (Yes, I imagine I could have planned it a bit better, but it was still full light when I got to the rim.)

Anyway, as I was making my way up, I was thinking myself about all the capabilities, both mental and physical, which degrade with age. Research appears to show that if you don't use them, they atrophy.

I wish I could take a hike like this at the end of every day. The scenery is a spectacular inducement to go for a walk.