Who knows what will catch the public's fancy?

The Pet Rock did; the Cricket House (complete with two live crickets) didn't. The Pluto Platter did, but only after the name was changed to Frisbee. Monopoly did, Trivial Pursuit did, Pictionary did; Park and Shop didn't.Past fads have included everything from CB radios to Wacky Wallwalkers, and such things as Rubik's Cube, Silly Putty, mood rings, Reebok walking shoes, white wine, miniature Japanese trees and kiwi fruit.

By classic definition, a fad is something that many people are interested in for a short time, a passing fancy, a craze. Some fads go on to earn a permanent place in our lives; others die fairly quickly. Fads are more transient than fashions, and both are less permanent than styles. All have a place in our lives.

Fads and fancies cover the gamut from what we eat, to what we wear, to what we watch and what we play with. Nor are they anything new. Back in great grandmother's day, the fads included such things as stitchwork samplers, hair wreaths and silhouette portraits.

The Futures Group, a Connecticut-based consulting firm that forecasts trends for both public and private clients, notes that fads spread because the public decides they are `in,' which means they are popular with trendy people. Media attention helps, too.

"Fads fall into two categories pure fun and functional," Ted Gordon, a Futures Group spokesman, told UPI. "Timing is everything."

Because fads and fancies are temporary things, they are not unimportant. Fads serve as a means of communication; they tell others that we think, what we like, who we are trying to copy. Fads can provide a psychological lift, a feeling of being with it or they can be a form of pressure. For teens, particularly, fads and the resulting peer pressure can be a big determinant of behavior.

Fads fill the need to belong and can even fill some newly perceived need, as the CB radio did.

But for the most part, fads and fancies are just something fun something to enjoy, to not spend a great deal of money on, something that keeps life interesting.

Some people make a career out of tracking what's "in" and what's "out." And not everyone agrees. But here's a look at a few of the things that are currently in vogue.

DAIRY COWS

Move over dinosaurs. Cows are the latest darlings of the marketing world. Cow figures are turning up on everything from mugs to T-shirts. There are fluffy Holsteins for hugging and wooden Holsteins to serve as doorstops. Pick up any direct mail gift catalog and just look at all the cow stuff: cow ties, cow sweaters, cow notepads, cow towels.

Advertising Age is calling this year a bull market for cows, noting that the gentle creatures are pushing aside the dinosaurs on the marketing merry-go-round.

According to Ted Tamarkin, designer for a New York-based giftware company, cow items turned up in droves at the recent New York Gift Show. A gift shop in Edgartown, Mass., called Everything Cows does a brisk business. The best-selling greeting card in Boston's Rainboworld Cards' line is a Holstein wearing hear-shaped shades.

And if cow products and cow bathroom accessories and cow socks are not enough, you can always go for the real thing. One of the newest ventures involves "adopt-a-cow." For the paltry fee of $250 to $400, you can adopt a cow on the 180-acre Panagakos farm in Hopkinton, R.I. "Parents" receive adoption certificates, are allowed to name their adoptees, can visit them once a month and throw them birthday parties. Sound far-fetched? All his heifers have been signed over, says Panagakos, and there are expectant parents on the waiting list.

It is maybe no coincidence that sales of dairy products are also up. The National Dairy Board reports that Americans are eating more cheese, scooping into more ice cream and drinking more milk than ever before. In fact, overall milk consumption has risen nearly 10 percent in the past three years.

All this attention to cows is not lost on the dairy industry, who are pleased that their "beloved cows are in such demand." It is, says a spokesman for the National Dairy Board, what you might consider a "moo-vement."

MEXICAN FOOD

There was "La Bamba." And the top-10 Hispanic music hits. And fashionable eating in Tex-Mex restaurants. Hispanic themes and images are invading pop culture as never before and perhaps nowhere as much as in food.

According to the U.S. Department of Commerce, the number of franchise restaurants specializing in Mexican food has jumped by more than 60 percent since 1980. And according to SAMI-Burke Inc., a New York market-research concern, sales of Mexican food in grocery stores from 230 percent between 1980 and 1987.

And if you want more proof that Mexican food has arrived, watch for new "lite" and "quick and easy" variations on menus and on prepare-it-at-home products. One company, Rosarita, for example, is touting a new "Quick, Light and Easy" cookbook that features all the traditional flavors of Mexican with the low-calorie and easy-fix twist of the '80s. (It's available, free, if you write to Rosarita Cookbook Offer, P.O. Box 080, Young America, Minn. 55399.)

Can the McChicken taco be far behind?

POPCORN

Popcorn is nothing new. It has been around since the 15th century, in fact ever since spaniards sent by Christopher Columbus returned from Peru with reports of an odd custom: "They toast a certain kind of corn until it bursts."

But popcorn has never been more popular.

Americans spent more than $440 million on unpopped popcorn last year, consuming an average of 46 quarts per person. Overall sales of popcorn, including that sold in movie theaters, was 693 million pounds. Sales of unpopped popcorn for home use have increased 92 percent in the past five years, placing it in the top 5 percent of fastest growing food categories.

And it's not expected to change in the near future.

Nor is ordinary popcorn good enough, any more. Nowadays, you find premium popcorn, select popcorn and gourmet popcorn.

So, what is responsible for this phenomenal popularity? Several things:

The growing trend among Americans to "graze" to eat small amounts of snack foods over the course of a day rather than three square meals.

The increased popularity of microwave cooking and microwave popcorn. By 1990, it is expected that 90 percent of all American homes will have a microwave. And microwave popcorn accounts for 44 percent of all sales.

Probably most importantly, the emerging recognition of popcorn as healthful food. Health organizations such as the American Cancer Society and American Dental Association endorse popcorn as a whole grain, high fiber, zero cholesterol, sugar-free snack as long as you don't smother it in butter and toppings. A cup of air-popped popcorn without butter and salt contains only 20 calories.

For popcorn fans, here's a bit more popcorn trivia:

The corn belt annually produces nearly 700 million pounds of popcorn. U.S. popcorn is shipped to more than 70 countries.

White popcorn pops smaller and is a bit sweeter than yellow corn.

18 tons to 20 tons of popcorn a day are popped and made into Cracker Jack (which has been around since 1893).

There are annual popcorn festivals in Valparaiso, Ind., and Marion, Ohio. Marion even has a popcorn museum.

HULA HOOPS

Remember 1958? Transatlantic jet service was inaugurated by Pan Am; the first U.S.-built commercial jet, the Boeing 707, was put into service. The first voyage under the North Pole was completed by the atomic submarine, Nautilus. The first U.S. satellite was launched. Khruschev was named to the top spot in the Soviet Union. The Chevrolet Impala was introduced.

That year postal rates climbed to 4 cents an ounce. Elvis was in the

Army. Arnold Palmer won his first Masters fold tournament. "Ozzie and Harriett" were bringing up David and Ricky, and Perry Marson was the king of the courtroom.

And the Hula Hoop was introduced. Not only introduced, but taking the country by storm. Within four months after the first hoops showed up in stores, 25 million had been sold.

The book, "American Fads," calls the Hula Hoop the "undisputed granddaddy of American fads. No sensation ever swept the country like the Hula Hoop. The hoop rewrote toy merchandising history during the summe of that recession-bound year, and it remains the one standard against which all national crazes are measured."

And it is coming around again.

Now in its 30th year, "we expect a major bump in sales this year," says Dan Roddick, sports promotion director of Wham-O, manufacturer of the hoop.

Hula Hoops appeared at the Super Bowl in January. In March, Disneyland visitors attempted to establish a world record for the largest number of people Hula Hooping at once. And it is an integral part of Disneyland's "Blast to the Past" celebration that runs through May.

Wham-O is also conducting a national video search for the best Hula Hooper in the country. (VHS videos up to three minutes long should be sent to Hula Hoop 30th, 835 East El Monte, San Gabriel, CA 91778. Winners will share $10,000 in prize money.)

One appeal of the Hula Hoop is nostalgia, says Roddick. But another harks back to its origins as an exercise device. Working out with the Hula Hoop can promote body flexibility, he says.

And so, round and round the Hula Hoop goes. And where it will stop? It just might not.