Lying on his side on the surface of London's iconic Millennium Bridge, artist Ben Wilson paints a piece of dried chewing gum trodden into the ground. "The important thing is the gum is below the metal tread," said 60-year-old Wilson, dressed in a paint-daubed orange jumpsuit. "The beauty of it is they're all different shapes and sizes so there's no conformity." What most people actively avoid or simply don't see, Wilson views as an opportunity to turn a tiny piece of discarded rubbish into something beautiful. It's also a way to delight passersby, enticing them to take a closer look underfoot. "By painting a picture which is so small, those that see it then discover a hidden world beneath their feet," Wilson said. "If they look then they see, so it's about perception.