Writing in the Gym

Well, okay, not typing on a computer, but soaking up people and their lives, like:
*The woman who's legally blind but whips along faster on the treadmill than I do.
*The older man who was Mr. Universe fifty years ago, is trim and fit and outdoes everyone else with his multiple body lifts and chin ups on the high bar.
*A young college couple that shows up 3x a week, works out together and leaves smiling.
*Moms of all ages who stow their preschoolers in the "babysitting room" but exit Zumba class in a trice if they hear a familiar howl.
*The young guy who pushes himself so hard, he drops his free weights with an exhausted clatter and startles everyone nearby.

I bet there's a book or subplot behind each of these people.
I need to keep track of them and think about them.

This is writing, too.