Mobs aren't new. Throughout history, crowds of people have gathered, often to protest. And the fun side of flash mobs has given us Michael Jackson dances at the Lisbon Airport, "Hammertime" at a mall in Los Angeles and a few hundred people gathering for a friendly public pillow fight.
But Temple University professor Frank Farley says the difference is the speed at which flash mobs can be assembled, the size, and whether they have a purpose.
"The more people that you have, the more likelihood something unusual can happen and it can spread," Farley said. "The biggest problem is the numbers. You know, and if it's organized around a theme, [a] performance of some sort, that's one thing. If it's ill-defined or undefined, then it leaves it up to people in the crowd perhaps to create something, and that can sometimes be bad.
"When anyone gets together and there's unpredictability and uncertainty, and there's a strong thrill factor involved here, and a lot of arousal and excitement. Sometimes it can spill over to the dark side and, almost by chance, almost by accident, something happens, and then other people pile on."
Philadelphia Mayor Michael Nutter said he'll institute a stricter curfew if necessary.
"Pillow fights. ... You want to have a pillow fight?" Nutter said. "I'll give out the first 100 pillows. You want to act like a knuckle-head? You'll be one of the first hundred arrested."
Nutter also said it's time for parents to take control. He said parental control extended to kids' text messages or Twitter posts.
"I'll just give you my own experience," he said. "When I was a kid, I didn't own anything in my house. I happened to be a guest. It was a room my parents let me use. I couldn't have anything in my room that was inappropriate. ... Many of the parents are paying for those accounts, so I think they have some right to the information.
"But parents need to know what's going on in their kids' lives. When they don't, unfortunately, sometimes there are bad consequences. They're still kids. That's what this is about."
Some Philadelphia residents voice frustration with their city for the violence.
"I'm trying to figure out why we in Philadelphia couldn't get it right," resident Sharon McWilliams said. "In every other city that has these flash mobs, it's all about the art scene. My daughter who lives in New York, all of a sudden 700 people converged at a certain time and then they all start to dance, or they all start to sing. I don't know how somehow Philadelphia didn't get it [right] again."
To be fair, Philly has "gotten it." There've been pillow fights and people freezing like statues at the 30th Street Train Station, and a dance on the steps of the Philadelphia Art Museum, made famous by the "Rocky" movies. None of those events went bad. Still, the mayor wishes he could harness all that flash mob energy and put it to good use.
"We'll all meet somewhere and rehab a house," Nutter said. "We'll all meet at the library and study together, help out on SATs or something like that. ... Go volunteer to clean up a neighborhood. There's always stuff. ... Two-thousand young people cleaning up a neighborhood, I love it. There are positive things that you can do."