When is cleaning walls a crime? When you're doing it to create art,obviously. A number of street artists around the world have started expressing themselves through a practice known as reverse graffiti. Inspired by the "clean me" messages that you see written on the back of some trucks,they find dirty surfaces and inscribe them with images or messages using cleaning brushes or pressure hoses. Either way,it's the same principle: the image is made by cleaning away the dirt.