The Strip of Grass Between Number Nine and Number Eleven

Between number nine and number eleven on Birch Close runs a strip of grass about three metres long, between the pavement and the road. It belongs, officially, to the council, which has not visited it since anyone can remember. Unofficially, it belongs half to Mr Okafor at number nine and half to Mrs Lindqvist at number eleven, and the dividing line is a crack in the kerb that both of them know exactly.

They were not close. Mr Okafor had moved in six years earlier and kept to himself, playing the radio quietly in his garage on Sunday mornings. Mrs Lindqvist had lived at number eleven for thirty years and had a sharp way of saying good morning that made it sound like a question. They nodded. They took in each other's parcels. That was the whole of it.

The first summer, Mrs Lindqvist mowed her half of the verge and, feeling generous and slightly guilty about a remark she had once made about his bins, carried on for an extra half metre past the crack. She did not mention it. The following Sunday, Mr Okafor mowed his half, noticed the neat edge and, not wanting to be outdone, mowed a full metre into her side. He did not mention it either.

It went on like that. Each of them crept a little further each time, early in the morning, before the other was up. By the third summer, each was mowing the entire verge every week, which meant that it was the best-kept strip of grass on Birch Close and quite possibly in the town. Passers-by assumed the council had got a new contractor. The council, when asked, was equally baffled.

In the fifth summer, Mr Okafor went away for a month to visit his daughter, and came back to find his front hedge trimmed, his bins washed and a jar of homemade redcurrant jam on the doorstep with no note at all. He knew exactly who it was, and she knew he knew. The following week, her gate, which had squeaked for a decade, stopped squeaking.

They still have not spoken about any of it. They still just nod. But on summer evenings they can sometimes be seen standing at their front doors at the same time, looking at the verge with the quiet satisfaction of two people who have been in a long, secret competition and have both, somehow, won.

The story is invented, but the habit at its heart is real. Psychologists describe a strong human tendency called reciprocity: when someone does us a good turn, we feel an urge to return it, and often to return a little more than we received. Studies of kindness have also found that doing small good deeds for other people tends to lift the mood of the person doing them, not just the person receiving them. Which may explain why a quiet exchange of favours can keep going for years, even between people who hardly say a word.