Banksy is Britain's most celebrated graffiti artist, but anonymity is vital to him because graffiti is illegal. The day he goes public is the day the graffiti ends.

His black and white stencils are beautiful, witty and gently subversive: policemen with smiley faces, rats with drills, monkeys with weapons of mass destruction (or, when the mood takes him, mass disruption) little girls cuddling up to missiles, police officers walking great flossy poodles, Samuel Jackson and John Travolta in Pulp Fiction firing bananas instead of guns, a beefeater daubing "Anarchy" on the walls. He signs his pieces in a chunky, swirling typeface. Sometimes there are just words, in the same chunky typeface - puns and ironies, statements and incitements. At traditional landmarks, he often signs "This is not a photo opportunity". On establishment buildings he may sign "By Order National Highways Agency This Wall Is A Designated Graffiti Area". (Come back a few days later, and people will have obediently tagged the wall.)

It is easy to become addicted to his work. Since spotting my first few Banksies, I have been desperately seeking out more. When I do come across them I find it hard to contain myself. They feel personal, as if they are just for me, and they feel public as if they are a gift for everyone. They make me smile and feel optimistic about the possibilities of shared dreams and common ownership.