On May 3, 2004, a mysterious artist known only as Banksy appeared in London with a new piece of street art. He had painted his own version of the Mona Lisa -- but with a bright yellow smiley face replacing her famous expression. The image appeared on a wall overnight. Nobody saw him do it. He creates art in public places without permission, usually under cover of darkness. His real identity remains a secret. Banksy's smiley-face Mona Lisa raised a question that his art always raises: is this art or vandalism? Painting on walls without permission is illegal. But Banksy's work has become so valuable that building owners sometimes protect his paintings behind glass. Some Banksy works have sold for millions of dollars. The smiley Mona Lisa was a commentary on how we treat famous art. The original Mona Lisa sits behind bulletproof glass in the Louvre Museum in Paris. Millions of visitors photograph it every year, often without really looking at it. Banksy's version asked: has the Mona Lisa become just a symbol, like a smiley face? Do we still actually see it? Street art like Banksy's challenges the idea that art belongs only in museums.