Hundreds of people march into a bookstore looking for a nonexistent book. Others break into simultaneous applause in a luxury hotel lobby. Another crowd heads into Central Park and starts making bird noises.
These pranks are the 21st-century answer to goldfish swallowing and phone-booth stuffing. So-called "smart mobs" or "flash mobs" organize online and plot sensational, usually nondangerous, stunts.
Science-fiction author Larry Niven is said to have inspired the concept with his 1973 short story, "Flash Crowd." Futurist Howard Rheingold, who popularized the term "virtual community" in the 1990s, promoted the idea with last year's book, Smart Mobs: The Next Social Revolution.
Technology makes it easy to keep hundreds of people in on the joke. Websites, cell phones, and e-mail help channel the energies of people with too much time on their hands.
The organizer usually tries to stay anonymous, adding to the anarchist nature of the mob. The exact plan is not revealed until the last minute. The group is told to meet at a certain spot, then members get instructions right before the stunt begins.
To outsiders, the flash mob appears out of nowhere, does something silly, and then disperses as quickly as it arrived. Websites with names like CheeseBikini and FlockSmart track these groups' goings-on.