I asked my upstairs neighbour three times to stop throwing used tea bags onto my balcony. The third time he laughed at me. So I stopped asking, and I spent an afternoon getting ready instead.

I never wrote his name on it. Not once. Not a flat number, not an initial, not a word about a person.

There was no heading. There was no explanation. There was no sign saying what it was.

It was not an accusation. It was a data set.

I put it up at twenty past seven in the morning, next to the washing lines, where all fourteen flats walk past on their way to the bins.

I did it fast and I did not look up at the balconies once while I was doing it, and my hands were going, which I had not expected at all.

And then I went to work.

By the time I came home there were four people standing in front of it.I want to describe the first day properly because it did not go the way people assume.

Nobody was outraged. There was no gathering. Two people thought it was an art thing.

A man from the ground floor told me later that he had looked at it for about a minute, decided it was a school project belonging to somebody’s child, and gone to the bins.

What changed it was the second morning, when I added the fortieth.

Because a board that is finished is a curiosity.

A board that gets a new one at twenty past seven, while people are walking past on their way out, is a live instrument.

By the third morning there were people waiting to see whether it would go up.

Because it was not the tea bags that turned that building.

It was the times.

7:03. 9:10. 7:04. 9:08. 7:01. 9:12.

Every day. Including Sundays. Including the twenty-sixth of December.

You only have to read that column for about four seconds before you stop thinking about tea and start thinking about a person who does the same thing twice a day, on purpose, for months.

And you only have to look up from the board to the balconies to know which flat.That is the movement I watched about thirty times over four days and it is always exactly the same.

Eyes on the times. Two or three seconds. A small stillness.

Then the head comes up, and it goes to the wall of balconies, and it stops on the fourth floor.

Nobody points. Nobody says anything. It is done entirely in the neck.

Mrs Ackerley from the ground floor stood there for eleven minutes on the second morning with her finger going down the column, and when she finally looked up, she did not look at the fourth floor first.