I don't have a 40' wall to play on, so I have created a pocket graffiti template. Ha! I did the mock-up on Styrofoam (left over from lunch, cut it with an Exacto, and sprayed a sample only to find out that spray paint eats Styrofoam! The sample was merely a model; I had planned to recut it in a sheet of copper (more durable), but wow! didn't know about eatability.
Maybe that's a way to destroy all of the Styrofoam clogging up landfills.
The image I decided on is an eye surrounded by a happy/sad, comedy/tragedy mouth. If you come upon it in the street you can see you have a choice-positive or negative, or it can be read simply as an eye. LOOK!
Materials: Styrofoam, spray paint + any surface, anywhere
