New Yorkers often become part of unexpected mob scenes - huge crowds on subway platforms, at clothing sales, or at free concerts in places like Central Park.
Now they are doing it on purpose, for fun.
"Flash mobs", in which people show up at an assigned place at a certain time, perform some brief acts and then leave, have descended on stores, a hotel and even a park in New York.
In the latest occurrence, about 200 people converged on a Central Park ridge across from the Museum of Natural History on Thursday.
Once in place, the mob tweeted like birds and crowed like roosters, chanted "Na-ture" and then dispersed.
If you are wondering what the point is, there is not one.
The gathering was the fifth installment of the New York-based Mob Project, which started in June with a guy named Bill who sent an e-mail to some friends, who forwarded it to their friends, and so on.
Bill, who declines to give his last name, aims to make the project last a few more months.
For him, it is a way to get people out, just like inviting them to a friend's play.
"The idea was to dispense with the event altogether and have the audience come together for no reason," Bill said.
Among the New York sites that have been mobbed are a Hyatt Hotel, where members spontaneously began clapping.
In Macy's, they pretended to shop for a "love rug" for their joint home.
At a high-end shoe store in Soho, they acted like tourists from Maryland.
The absurdist idea is catching on outside New York too.
Bill says the Internet has been used to organize "flash mobs" in cities like Boston, Minneapolis, San Francisco, London, Rome and Vienna.
Four-time New York mob participant Theodore Grunewald was among those who received a Mob Project email advising them to synchronize their watches and go to one of four Upper West Side bars on Thursday evening.
Once at the bars, the would-be mob members were given flyers telling them to assemble in Central Park at a certain time, make bird noises, chant, cheer, and leave.
Mr Grunewald described the mobs as being urban poetry with no real purpose.
"It turns New York into an enormous toy," the 36-year old said. "And it brings all sorts of people together unexpectedly."
http://www.abc.net.au/news/newsitems/s910802.htm