One evening a few summers ago, I convinced a friend to run with me up Portobello Road completely naked. As we reached the finish line, we could hear the sirens in our wake. We were accosted by two policemen. I was convinced they would throw us in the slammer. Instead, the officers gently told us that it would be wise to put our clothes back on. One of them, having seen my body gleaming in the pale moonlight, suggested that I should really consider getting a tan. I have long enjoyed streaking. People wonder why someone would choose to expose themselves in such a way. There are a few explanations. Public nudity is often an act of rebellion or defiance — see John and Yoko back in the 1960s. More recently, supporters of the Chinese artist Ai Weiwei have stripped off in solidarity with him.