When Philadelphians think of "flash mobs," they now think of large groups of unruly teens running amok. But that's not how most of the world sees this internet-organized trend. On Thursday evening, a much more peaceful "mob" converged on Philadelphia's 30th Street Station. Hundreds of participants froze in place when the station clock struck 6:30 p.m. and they stayed frozen for three minutes. The word went out on Facebook. "Usually, it's a very positive thing," said Temple University psychologist Frank Farley. "Like kids get together and do some street art."