The poo shoe had become something of a local cause celèbre

The tall blonde woman with whom I share this flat, a queen-sized bed and a familiar creeping anxiety about where our teenage children have got to, stepped in something nasty the other day. She often goes for a health-restoring amble on Hampstead Heath, which has long since become a sort of vast, extended dog toilet, attracting incontinent canines and their gleeful owners from miles around.

I knew something of the sort had happened when I came home and saw the offending boot on the doorstep. She hates having to clean poo off her shoes, and who doesn't? One of the most marvellous moments in a parent's life is when you realise you have wiped your children's bottoms for the last time. Not only can they now do it for themselves, but they prefer to, and you are officially off the hook. Years later you might feel slightly sentimental about this and remember the duty with a certain nostalgic fondness. But you will be wearing rose-tinted spectacles, as well as a rose-coloured peg on your nose.

As I write, the boot is out there still. It's only five days, but the tall blonde woman figures that no one is likely to steal a single boot, especially if it is tainted with the muck of hounds. Out of sight, though, means out of mind. When she goes in and out of the house, I don't think she even sees it. Buddhist monks practise mind-control techniques for decades to reach this state of grace. I could offer to clean the boot myself, but obviously I'm not going to do that.

The thing is, it's not the first time this has happened. The other night I was coming back from the pub with my friend and neighbour Alan, who spotted the solitary footwear item immediately. "The return of the poo shoe," he said, portentously. It's not actually the same one: the previous incumbent was a daggy old trainer that sat on the step for a while, about a year ago. "Do you remember that, then?" I asked. "The poo shoe?" said Alan. "Of course. Everybody remembers the poo shoe." I thought it had been there for a couple of weeks, maybe a month at most, but Alan said it had sat there for at least three months. "Funny," I said, "you never mentioned it." "Well you don't," said Alan, miming a telephone call. "Hello Marcus, it's Alan. How are you? And did you know that there's a shoe with poo on it that has been sitting on your doorstep for a quarter of a year?"

Apparently the shoe had become something of a local cause celèbre. Some of our neighbours began to find its presence oddly reassuring. Its apparent permanence contrasted favourably with the constant change and disruption of everyday city life. Go out in the morning to work, pass the poo shoe. Come back home that evening, poo shoe still there.

She did clean it eventually, of course, by which times its doggy contribution had dried to a fine powder. It didn't even smell any more. I am not sure the shoe was strictly wearable after it had been out in all weathers. But Alan said that people missed it. They wondered what had happened. For them this was a story that hadn't ended satisfactorily: it had just stopped, in the middle of the last chapter, without explanation. I had no idea of any of this.

And now there is a poo boot out there. Word is getting round. Tourists will soon be flocking. Perhaps we should charge admission. 10p to look, 50p to touch, £5 to clean the boot.