Flash mobs initially involved dancing in the streets, freezing like statues or even giant pillow fights.
But then something else happened. Bicyclists in San Francisco and Los Angeles started blocking traffic to protest lack of respect from motorists. A spontaneous snowball fight during the winter's epic snowstorm in Washington, D.C., went sour when a plainclothes policeman pulled out a gun.
And, then, two weeks ago in Philadelphia, touristy South Street was the scene of a flash mob gone bad. Foot traffic on the street changed in minutes from normal Saturday date night to an onslaught of mostly young people seemingly intent on destruction.
Tom Vasiliades, owner of South Street Souvlaki, caught some of the action, both inside and outside the restaurant, on surveillance video.
Startled customers huddled inside. Someone locked the doors.
"The people, we locked the doors and they stayed here until the streets were under control," Vasiliades said. "Everybody was afraid. ... Really afraid, yes. Customers and everyone."
Noel Meneses, who manages a pizza parlor down the street, said the crowd was out of control.
"What I see was like young kids getting on top of the cars that were parked on the side of South Street," Meneses said. "They were jumping on top of the cars, smashing windshields, breaking windows. ... My customers got so scared, so what I had to do is lock all the windows and the doors and not let anyone in or out for at least half-an-hour. They were so scared and these kids started to come in. ... I've never saw that before. First time in my life that I see something like that."
Vigilant Parent Tips Off Cops
Police had received an anonymous tip from a parent, so they were ready. They quickly dispersed the crowd but the incident frightened customers and business owners.
"On weekends, we close at 11 [o'clock], and I had to close at like 10 because the police took everybody out of the street and kicked them out," Meneses said. "There was nobody on South Street after 10. ... They had moved all the kids."
Philly Flash Mobs: 'A Very Tough Stand'
Having spent the past two years fighting to free Philadelphia from its nickname, "Killadelphia," and lower the crime rate, city officials, including Police Commissioner Charles Ramsey, are taking a hard line against violent flash mobs.
"Judge [Kevin M.] Dougherty, who sits on Family Court [Juvenile Division] here, took a very strong position," said Ramsey. "He's my new hero, because he really took a very tough stand on these kids. We don't want to give them records, we don't want to, or do anything that will harm them in the long run, but at the same time you can't just assault people.
"You can't destroy property of others. Come down and enjoy yourself, but you can do it without causing a lot of damage and a lot of ... issues for business and others."
It wasn't Philadelphia's first brush with the ugly side of flash mobs.
"It actually wasn't on South Street the first time we encountered it," Ramsey said. "It was downtown, in what we call Singer City, where after school, through messaging, kids decided to gather in one of the shopping complexes called Gallery Place. They were put out of there because there were so many of them and they were a little disruptive, but then they started to go through the streets and knocking people over. They went in the Macy's Department Store and caused some issues and problems. We did make a few arrests down there."
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."
Philly Flash Mobs: Parental Responsibility
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."