Photographer Spencer Tunick persuades people to strip naked in public places. These odd gatherings have attracted up to 7,000 participants, who for one day in their year, or possibly their lives, get to do something out of the ordinary.
The photos are impressive but, from an exhibitionist's point of view, they must be disappointing. The mass of bodies packed so closely together makes it impossible to identify anyone.
The anonymity is one of the great attractions of joining in a Tunick happening. By taking part, you can do something outrageous in public, but still enjoy the comfort of being part of a crowd. It's the same with chanting at a demonstration, swearing at a football match or pirouetting around a McDonald's paper towel in a flash mob.
Most of these events have a purpose. Tunick's human installations are part of his art, demonstrators hope to change society, and football fans try to intimidate their opponents. But flash mobs are pointless.
As we have all read a thousand times, the flash mob phenomenon first appeared in New York in June. They began as an act of spontaneous fun; an old prank using modern technology.
There's nothing new about organising events by e-mail or text message. The name came from two science fiction authors who wrote about the use of technology to organise and assemble rioters. Larry Niven described Flash Crowds in 1973, while Neal Stephenson coined the term Smart Mobs in 1995. In recent years, many of the protesters at anti-globalisation events have been described as smart mobs, as they plan events by e-mail, publicise them online and co-ordinate on the ground by mobile phone.
At first, this worldwide wave of pointless pranks sounded like a marvellous idea. Flash mobs were silly, but that was the point: they were pointless but fun. Arriving in the traditional 'silly season', when newspapers have little better to write about, flash mobs were whipped up into a global trend, and began forming in every large city in the world.
Nobody has dared to try organising one in Beijing yet, but give it time, and someone is sure to try.
Inevitably, Hong Kong has caught up with the craze, and both groups to have attempted it so far have looked hopelessly confused. Hong Kong's first flash mob, which mobilised in Causeway Bay on Friday, apparently chose that day just so they could beat Flashmob@Hong  Kong, which had planned to hold its first mobbing on Sunday.
Both groups publicised their events online, and both ran exacting instructions as to what was to come. Where's the spontaneity in that? The Sunday mob was asked to shout out a radical cry of 'I love Hong Kong!' before 'escaping' into the Tsim Sha Tsui crowds - presumably to avoid angry tourists waving overpriced cameras.
The Friday mob's remarkably complex plan involved assembling according to birth date, impersonating airplanes, pointing at the sky and applauding the Times Square clock tower. And just so we would all know what to expect, the instructions were posted online three days in advance.
By publishing what was to take place, any spontaneity was impossible. Yet when the details were reported in this newspaper, organisers complained. But if you don't want your party publicised, don't advertise it. In the event, the small gathering was reconvened at a nearby McDonald's - where the press was waiting, thanks to the new instructions posted to the mob's website.
In internet time, a new craze can hatch and die faster than a jar of Sea Monkeys. Rather than being wacky and spontaneous, flash mobbers have become jaded and predictable. And just like the lethargic masses they set out to startle, they are no more rebellious than Avril Lavigne.
A group in Birmingham, England, held the first meaningful flash mob last week, when they turned up at a local branch of Oxfam, sang the Red Hot Chili Peppers' song Give it Away, and donated their clothes to the shop.
The mob then announced its dissolution, saying: 'There has only ever been one flash mob. It happened in New York in early June. The rest have all been imitations. To all the other mob organisers around the world: give the idea the freedom to grow. Let it evolve or put it down.'
Posting to the forums on Hong Kong website Uzone21, someone named Calvin expressed similar sentiments: 'They should do something more meaningful, like getting together on a beach and pick up all the trash and garbage and then disperse'.