It was a queen of diamonds sandwich. There was the old girl, an ace of spades above her, an ace of clubs below her."You know where the queen is," said the smooth man holding the three cards face down. "Pick her out for me."

The victim's hand moved to the middle card. Whoops! The ace of spades. So it had to be the top card, then. Whoops! The ace of clubs. "So it must be the other card, right?" said the man. And he flipped over . . . the ace of spades.

Steve Mathews is used to the bemused laughter that follows. He is even more used to the question that he gets from young and old: "How do you do that?" His reply, always delivered as if he has just swallowed a canary, is: "It's magic."

It is more like hard work, much practice and years of experience. But Magic by Mathews is more than a business card. "This is a habit worse than [WORD ILLEGIBLE] or heroin," says Mathews, a promotional magician for four years. "I love it more than ever. I plan to do this when I retire."

From being a detective.

Yes, you students of life's unlikely pairings, Mathews the Magician is also Detective Stephen R. Mathews, Sex Squad, Metropolitan Police Department.

While many of his police cohorts choose hobbies that get them as far from police work as possible, Mathews sees a great usefulness for magic in his police work.

He admits that his squad mates "think I'm a hot dog." But then again, it's hard to argue with what happened in Southeast Washington last summer.

Mathews was walking a beat near the Navy yard on one of his days off, to supplement his income. It was a killingly hot day, so the kids playing basketball and jumping rope were glad for the respite when Mathews strolled into the playground and said the magic words: "Pick a card, any card."

An all-day magic show followed. It was actually a moveable feast. Mathews had to leave every hour for a quick hike around his beat, but the kids followed. "I felt like the Pied Piper," he said.

Mathews thought nothing of the experience until one evening several months later. A rape occurred a block from the playground. Mathews came to take the report.

The victim had no idea who her attacker was. But just as Mathews was leaving her apartment, a youngster strolled up to him.

Hey, aren't you the cop who does the magic tricks?" the boy asked. Mathews said he was. "I know who did it," said the boy, pointing down the block. An hour later, Mathews had made the kind of arrest that only happens on television.

Less dramatically or conclusively, magic helps Detective Mathews settle down children he must interview.

"I show a few tricks to a child who's been abused, and it always helps them relax," he said.Perhaps most important, magic "takes away from the I'm a policeman' kind of image that scares people."

Mathews' earliest image of magic came from his father, a career Army man. "He showed me my first trick, the toothpick in a handkerchief that disappears," Mathews recalled.

Magic gave way to music while Mathews attended Fairfax High school in the 1960s.He played guitar in a rock band called "The British Walker" and was planning to make music a career when he was drafted and sent to Vietnam.

"Over there, I didn't do much magic.Over there, I dodged a lot of bullets," Mathews said. But he admits he garnered some spare cash from his bunkermates - with the old standby, the vanishing queen of diamonds trick.

Mathews, 31, joined the police department in the summer of 1968. One day that fall, he walked past a magic shop near the White House, and the old coals were kindled. He has been performing for groups, parties, clubs and classes since 1974 and has taught magic at the Loudoun Campus of Northern Virginia Community College since 1977.

Mathews is the most maddening of magicians. His specialty is sleight-of-hand, usually involving sponge rubber balls, coins and cards. His props typically disappear, multiply or change into something else, right under your nose.

Mathews claims that if you watch him closely for an hour, his act will be an open book. But on Easter Sunday, the customers of a Falls Church restaurant watched him table hop for twice that long. Perhaps their bonnets blocked their vision, but not a soul figured Mathews out.

Wincingly enough, the restaurant was a creperie called The Magic Pan. So no one seemed surprised when Mathews introduced himself as "the resident magician" and offered to fill the minutes before the food arrived.

When Mathews put a 50-cent piece in the closed fist of Michele Geiger, 6, and then turned it into a penny, the squeals could be heard four tables away.

When Jenny Kuehnle, 10, picked the king of spades from a deck of cards, and Mathews retrieved it after lots of shuffles and cuts, she slapped her cheek as if she were swatting a fly.

Even the waitresses were stunned when Mathews asked a victim to pick a card and write his name on it. Not only did Magic Steve find it again, but after a few shuffles, he laid the full deck down on the bar, pulled out his billfold and produced the card - with the victim's name on it - all over again.

So beware, world. Not only is Steve Mathews good and getting better as a magician. And not only is Steve Mathews rewriting the police handbook on how to be a detective. But there is another star on the horizon.

He is Travis Mathews, 3. His father, stuffing three spongeballs that used to be one back in his pocket, said, "He's already better than I am."