He hits the top stair.

1

Then, he falls.
His field of vision fills with sky. Coffee splashes. Papers fly. His body takes formation midair in the shape of a jagged-V, and a small, birdlike sound comes from his throat. For the briefest of moments he is motionless. A tug from below, and his path changes.
His left heel touches down directly upon it. Rather than flattening into position, his foot slides forward, carried by the slip of the peel. It moves quickly and smoothly and efficiently. His centre of gravity fails to shift, holding still in the pelvic area. His right foot continues its transfer from heel rise to toe off, heading toward swing. His weight stays back. His right arm shoots up. His left arm reaches out. His left leg straightens, his foot lifts off and out, and he jackknifes at the waist. All the muscles in his body stiffen at once. With his left leg parallel to the ground and at right angle to the rest of him, his upper body torques, his shoulders tighten and his neck jerks hard to the side. His right foot toes off. He is airborne.
It probably had been there a few days. It may have been tossed from a car window, the remains of a quick snack enjoyed in transit. Or from the hand of a careless child. Perhaps it fell out of a bag. But there it lay rotting, the humble peel of a old banana, brown and black and flecked with yellow.
Only arthropods and vertebrates have the means of rapid surface locomotion. In both groups, the body is moved forward by means of the legs. Because legs provide support as well as propulsion, the sequence of movements must be adjusted to maintain the body’s centre of gravity within a zone of support; if that centre of gravity falls outside this zone, the body loses its balance and falls. It is the necessity to maintain stability that determines the functional sequence of limb movements. During propulsion – which begins with footfall and ends with liftoff – the foot and leg remain stationary as the body pivots forward over the leg. During recovery – which begins with liftoff and ends with footfall – the body remains stationary as the leg moves forward. The advance of one leg is a step; a stride is composed of as many steps as there are legs. To ensure a cycle of continuous movement, friction-reducing properties of a stable surface (ie. the ground) are needed to enable the appropriate and continuous transfer of weight from heel to toe.
It has been hot. It has been sunny. There has been a little bit of rain. He is wearing new shoes. He is heading east on a quiet road. There is a light dew on the ground. At first, he avoids the grass. He walks to the end of the street. He crosses the road, turns left, and walks a bit further. Giving way to a woman walking six dogs, he cuts across a freshly mowed lawn. He steps back onto the pavement. He is thinking about something. He does not see it. No one sees him.