The old house has a beech threshold with a hollow worn into the middle of it.
He has lived there alone for ten years.
Once a year he measures the depth with a vernier caliper and writes it on the inside of the door frame. First year to tenth: about a millimetre a year, very even.
He has done joinery and knows what that means. Beech is hard. To take that much off it you need tens of thousands of crossings.
He worked his own out: out and back twice a day, four crossings, about fifteen hundred a year. Ten years, fifteen thousand.
The depth of that hollow corresponds to something nearer sixty thousand.
He spread a thin layer of stove ash across the threshold, level, edge to edge.
In the morning he opened the door and looked.
There were prints in the ash. All of them going out. Not one of them coming back.