Two front doors face each other across a metre-forty landing.
The mat outside his door has turned round by the time he leaves in the morning. Brown, rectangular, bound at the edges.
He drew an arrow on the underside with a marker, pointing at his own door.
The next day he turned it over. The arrow pointed at the door opposite.
He bought four self-tapping screws and fixed the mat down through the grouting at each corner.
The next day the screws were still there, all four. The mat had turned on them. There was a torn hole at each corner, the fibres pulled out in the same direction.
He bought a new mat and did not fix it.
The next morning the new mat faced the door opposite.
He looked up. There is no mat outside that door.
There is a rectangle on the tiles where the colour is darker, worn in over years, with a clean edge to it. He crouched down and measured it against the tiling.
Two and a half tiles. Exactly the size of his.