The term "flash mob" used to mean online-organized groups having street-corner pillow fights or sidewalk dance-offs, but in Philadelphia they've turned violent. In the most recent mob, witnesses estimated that about 2,000 teenagers jammed sidewalks, blocked traffic, jumped on cars and roughed up bystanders "Social phenomena have to start somewhere," said Frank Farley, a psychologist and professor at Temple University. "It could be some kind of coincidence of fate that will get picked up later on elsewhere."