Given the monumental influence graffiti art has had on our popular culture, from music, film, and television to fine art, toys, and clothing, it’s easy to forget the form’s humble roots and remarkable evolution -- how what started as a way for bored kids to pass the time grew into a movement larger than anyone could possibly have imagined. Indeed, long before the giant murals, fashion runways, larger-than-life art shows and unlikely street art millionaires, modern graffiti art got its start in the belly of a Philadelphia juvenile corrections facility, with a single word scrawled in...