"If you want them back," someone once asked, "why throw them away in the first place?"

He was referring to boomerangs, or 'rangs, as they are known to the rapidly growing number of aficionados in this country and around the world.

A throwback to the Stone Age (aboriginal boomerangs have been carbon-dated at 11,000-14,000 years old), boomerangs are providing "many happy returns" to Space-Age fanciers, who count among their achievements:

*Throwing a boomerang from New York, across Niagara Falls into Canada and back into the United States.

*Tossing a boomerang from one year into the next -- launching it a few seconds before midnight on a New Year's Eve, with a New Year's catch.

*Throwing a boomerang (in Waiyev, Fiji) from yesterday, across the International Dateline into today, and back to yesterday. Or from today, into yesterday and back to today.

*Launching a boomerang from New Mexico to Arizona, to Utah, to Colorado and back to New Mexico.

"Boomerangs," insists Washingtonian Benjamin Ruhe, considered by many as the father of the current boomerang boom, "are the thinking man's Frisbee. Here's a sport where you can play catch with yourself." Ruhe, honorary boomerang consultant to the Smithsonian Institution's Air & Space Museum, says his own interest in boomerangs came during his stint as a Jackaroo (cowboy) in New South Wales in the 1950s.

Throwers are unanimous in their enthusiasm, although they voice it in different ways. Says Eric Darnell, coauthor with Ruhe of Boomerang: How to Throw, Catch and Make It (1985, Workman Publishing, $9.95, which includes a Darnell-designed polypropylene 'rang), "It allows you to think like a bird -- in terms of wind velocity, wind direction, humidity and temperature."

Enthusiasts and dealers alike agree that the growing interest in boomerangs is neither a fad nor a craze, and they're happy in that knowledge.

"It's a gradually building thing," says Richard ("The Boomerang Man") Harrison of Monroe, La. "If it were a craze like the hula hoop or that sort of thing we'd soon have to find another business to be in."

Harrison says he's received requests for his boomerang catalogue from all over the world, with a number of letters simply addressed to "The Boomerang Man," Monroe, La. His sales, he says -- no figures, thank you, but "You can say thousands a year" -- increase every year.

And so do the number of competitions and informal throw-ins. Yale University's Peabody Museum, for instance, began sponsoring a boomerang seminar, workshops and throws in 1983. According to museum director Leo Hickey, attendance "has about doubled every succeeding year. We started with around 50 and the last program drew close to 300."

Boomerang sales have also doubled over the past several years, notes market analyst David Boehm. "Boomerang sales probably totaled around $1 million in 1981," says Boehm, 43, "and this year sales should be around $12 million to $18 million." That translates to well over 1 million boomerangs sold in 1985.

Boehm's personal connection with 'rangs goes back to the '70s when he attended the Sydney, Australia, Boomerang School. Eight years ago he formed the Cleveland Boomerang School -- "free lessons every Sunday and 25 to 50 public demonstrations each week."

"We stopped counting when we reached 25,000 people taught," says Boehm, "but I'd guess we've trained around 30,000 people over the last 8 years." The financial return comes in boomerang sales to school participants.

Another enthusiast, Ted Bailey, designs and makes high-tech boomerangs when he's not working as an engineer with Teledyne Corp., Toledo, Ohio. Because he makes his boomerangs by hand, production is limited. "I sold around 50 when I started 5 years ago," says Bailey. This year he expects to turn out between 1,000 and 2,000.

Bailey says there probably are 2,000 to 5,000 "hard-core" enthusiasts in the United States; casual throwers are likely to number in the hundreds of thousands.

Local boomerang outlets say sales are brisk and steady. Chuck Bernstein, owner of The Kite Site in Georgetown, says sales "are very good. We mostly sell to people college age and into their twenties and thirties." While he won't talk dollars and cents, Bernstein says his shop sells "several thousand" 'rangs at retail every year and "many more than that" wholesale. "Abercrombie & Fitch," he says, "bought over 2,000 this year."

Predicts Bernstein: "You'll even be seeing more and more night-flying, boomerangs with Cyalume lightsticks inserted in them." People can modify their boomerangs themselves or Bernstein will do it in the store at $1 per 'rang. The Kite Store carries around four dozen models, ranging from $6 to $30.

Wendy Talbot, a saleswoman at Australia's Thingummybob, Alexandria, says that store has sold "well over 100" 'rangs since opening 13 weeks ago. It stocks four or five designs running from $9.50 to $12.95.

Alison Wood, co-owner of Australia Design Shop, National Place, says her store carries a dozen models, going for $4.95 to $24.95. "We've sold 500 to 600 since the store opened in April," she says, adding, "We almost can't keep them in stock."

Boomerangs also are available in many local toy and general merchandise stores as well as by mail-order.

"When you throw these things," says Brown University senior Andrew Shaindlin, 21, "you feel like you have a friend. It's very forgiving. No matter how hard you throw it, it always comes back."

Why? "Because," says Ruhe, "it has an airfoil to provide lift, because its spin causes it to act like a gyroscope, and because its spin and forward motion combined cause it to turn, or process, to the left."

Simple? Yes, say Ruhe and other experts, if you start with a decent boomerang and if you are willing to put in a little effort.

Among organizations for 'rang fanciers:

*The U.S. Boomerang Association, single memberships, $10, include a quarterly newsletter, Many Happy Returns, and other benefits. USBA, Ray Rieser, Box 2146, Lower Burrell, Pa. 15068.

*Maryland Boomerang Society. Dave Robson, 309C Burke Ave., Towson, Md. 21204.