They appear as if out of nowhere -- dozens of twentysomethings with instructions they get from a Web site and cell phones at the ready about to do... something. Flash mobs, semi-spontaneous gatherings of people akin to a practical joke, have sprung up in cities around the world.

Tuesday, 75 people clogged a Dupont Circle bookstore for no other reason than they could.

Interviews: Lloyd McCoy, 24, a Univ. of Md. graduate student;
Kavi Thakrar, 21, an intern from London;
and 16 year-old Molly McArdle, a junior at the Duke Ellington School of the Arts, who works at Books-a-Million

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