Around eight in the morning, the sun climbs high enough to reach the small window in our front door. Our house faces east, and for a short stretch the light comes straight through and lands on the living room rug, where my son and I start every day.

He is seven months old and learning to sit. I prop him in the middle of his play square, among the rattles and a cloth book he prefers to chew, and he wobbles, catches himself, and tips sideways into my hand. Then we switch to tummy time, and he pushes his chest off the floor with his whole face clenched.

One morning I looked up from him and saw the air.