If you notice London looking a little artier this weekend, here's why: The inaugural Cans Festival kicks off tomorrow. In essence, a bunch of the world's best-known graffiti artists (including some guy named Banksy) are coming to paint the town red—and, we suspect, a number of other colors—during a three-day street-art battle. Admission is free, but be warned: One of their "rules of engagement" is "We have a lot of security."