Okay trivia buffs, what is one of the fastest growing collector's items in the United States today? What single item has people sifting through old dumps, is enjoyed mostly by kids (with their parents inevitably joining in), and requires more space than any coin, stamp or butterfly collection? The answer: beer cans.

More than 200 beer can aficionados from the Washington area gathered Saturday at the Bethesda Community Center to trade, buy, sell and just plain admire thousands upon thousands of beer cans that range from the "historic" cone tops of the 1930s to the "J. R. Ewing" flattop pull-tab model introduced by the Pearl Brewing Co. of Texas late last year. p

Among a few husband-and-wife teans present were John and Polly Vetter of Fairfax. The Vetters, who have been collecting beer cans for the past 15 years, have progressed far beyond the "hobby" stage. They recently excavated a basement under their home for the sole purpose of accommodating their collection of more than 6,000 cans. "It's one of the few collectables where kids are on an equal basis with adults," said Polly. "And the historical aspects of the older cans also serve as an excellent teacher."

John founded the Washington area chapter of Beer Can Collectors of America. The national group has more than 10,000 members, a bimonthly magazine and its own convention. He says serious collectors need to be patient and observant.

Father-and-son teams seem to be particularly effective. Steven Rogers, 15, of Silver Spring, has been a collector for six years, and now his father Morris, 52, claims "he's got his daddy hooked, too." The Morris collection numbers 2,400 with an emphasis on quality cans. Robert Humenansky, 13, also of Silver Spring, started his 1,800-can collection when his father gave him 12 cases of empty beer cans for Christmas in 1977. Humenansky says, "I knew I was being drawn in (to collecting) when I found myself hauling six cases of empty beer cans back from a vacation in Majorca."

People who might pooh-pooh beer can collecting and its followers should note that in a May 1980 auction, a Tiger brand beer can from the Manhattan Brewery in Chicago sold for $6,000. Another can labeled "Rosalie Pilsner," brewed in Louisiana, sold for the same price. c

John Vetter says the value or "quality" of a can depends on three factors.

The first is rarity. Vetter points out that some types of cans -- such as those for Viking beer, made in Pensacola, Fla., in 1960 -- are so rare that only three or four exist.

The second is beauty. The Tiger beer cans, with their namesake painted on the front, are among the most distinctive, Vetter says.

The third factor is mystique. Cans with this quality include the "camouflage" cans produced by the Ballantine company of Newark during World War II. The cans were painted olive green, supposedly so the enemy couldn't spot them if soldiers dropped them in the jungle.

Polly Vetter gives these tips for starting a beer can collection:

First, build up a collection of as many cans as possible, both foreign and domestic. You'll get duplicates that can be used as "traders" for cans you really want or need. As your collection increases, you'll acquire finer-grade cans that also can be used as "sellers." From there, you're on your way to filling up cases, rooms and closets. You might even have to dig a basement to hold them all.

Bob Tupper, also of Silver Spring, has another requirement. Tupper and his wife Ellie have been collecting beer cans for 12 years. Bob says, "A can will not go into my collection unless I drink it." To date -- for his hobby's sake, mind you -- Tupper has emptied 2,300 cans.