They sway to the samba. They wiggle to the Watusi. They bop till they drop or at least until you turn off the sound. And since their introduction at New York City's F.A.O. Schwarz this spring, more than a million Rock'n Flowers—garishly colored, battery-operated, plastic plants—have danced their way past cash registers to become the year's most whimsical winner.

Already East Coast department stores and California boutiques are complaining they can't keep the faux flora in stock. By fall, the buds—which respond to music and other sounds by moving in their pots—will be gyrating in the Midwest. And unlike such predecessors as Hula Hoops and Pet Rocks, Rock'n Flowers come with a hefty price tag—$29.95 each.

"It's a perfect grandmother's gift," says a grandmotherly sort, surveying a Rock'n Flowers display at Bloomingdale's in New York with a smile. "Every person sees something different," says the store's housewares buyer, Hank Reinhart, "but they make everyone giggle."

Getting the ultimate laugh is Japan's Takara Co., which started manufacturing the flowers in 1988, tucking battery-powered audiosensors inside each pot. The sensors respond to noise of any kind—applause and whispers as well as Springsteen and Bach—activating a tiny motor that causes the flowers to wiggle. As a sound moves closer or increases in volume, the flowers move faster. As it moves away or becomes fainter, they slow down.

The flowers—outfitted with sunglasses and a variety of musical instruments—come in six color combinations and four varieties: hibiscus, sunflower, daisy and orchid. "Each flower has its own personality," says Scott Ramsay, operations manager for the trendy East Coast DAPY gift stores, where more than 10,000 Rock'n Flowers were sold in eight weeks. "Buyers choose those that suit their needs."

Often, the need is for a gift for a bedridden friend. "We've even gotten calls from hospital florists who want to carry them," says Jack Miller, vice president of WACO Products Corp., which distributes Rock'n Flowers in the U.S. But other buyers may simply be in the market for an exuberant companion.

"People always say you should to talk to your flowers," notes Miller. "Well, these flowers really respond."