I love the concept of "reverse graffiti". Simply put: instead of using spraypaint, find a dirty city surface and use a rag and some water. Create art that instantly doubles as social commentary (Why is our city so filthy?), and befuddle the cops who want to arrest you, but aren't sure if selective cleaning is a crime. Above, Brazilian artist Alexandre Orion cleans up a Sao Paulo tunnel (in summer '06). See a video of the piece's creation, complete with befuddled cops, here.