In Dortmund, Germany, a mob gathered by a washing-machine display in a department store, ate bananas, then left. In New York City a crowd assembled in Central Park and chirped like birds. In San Francisco they made like whirling dervishes on Market Street. Epidemic lunacy? No, just flash mobs—groups of strangers recruited by anonymous organizers through e-mails and Web sites to congregate in a public place, act wacky and then disperse. Don't look for a higher meaning; there isn't any. Part performance art and part prank, the fad, which began in Manhattan two months ago, is all about having fun and confusing bewildered onlookers. "It's like people watching taken to an extreme," says Molly Peeples, 19, an MIT astrophysics student who joined a July 31 mob in Cambridge, Mass., that shopped en masse for greeting cards. "It's playing with people's minds."