I was, until recently, blissfully unaware of my body’s mass. Gravity? Not really an issue. I moved through life with reliable and reassuring weightlessness. High hurdles? I glided over them in high school, despite the handicaps of being 5-foot-7 and having legs far too short for my ambitions. Soccer? Even as I played through my 40s and well into my 50s, I could accelerate, decelerate, cut, and pivot on instinct, without thought or effort. Not so long ago I still annoyed my wife by bounding into the air and clicking my heels together, just because I could.