If you were on a river boat and a stranger invited you to join him in a "friendly" game of cards, you would probably say, "No thanks. Go find yourself another sucker."

On a Metrobus, however, it's a different story. Three Card Monte games take place on the back seats of Metrobuses and there appears to be no shortage of suckers. Perhaps that's because a bus doesn't look like a river boat and monte tossers (card manipulators) don't look like crooks.

If you ride buses, please read today's column with some care. And remember what you have read.

Three Card Monte appears to be simple game, but it is neither simple nor a game.

Three cards are used -- usually the two red aces and the queen of spades.The card shark show the three cards to his audience, which always includes several persons who are the shark's secret partners.

The monte tosser throws the cards face down on any surface that is handy. Even a newspaper spread across his lap will do. He challenges you to pick out the queen and offers to bet even money you can't do it.

An unsophisticated person who sees this activity is tempted to bet. True, he's being offered even money on a wager that appears to be 2 to 1 against him. But, goodness gracious, the man with the cards shows all three of them before they are thrown face down, and the sucker is sure he can remember which one is the queen. In fact, some of the other men also have good eyes. They are betting and winning.

So the chump begins to bet. He may even win a few times.

He doesn't know that the people who won before he got into the game are the monte tosser's "outside men" (confederates), or that the entire group will meet later to divide up the money he loses.

Nor does he know that his chance of winning a bet is zero unless the card manipulator wants him to win a few small bets that will build him up for total disaster at the end.

There are many tricks and skills involved. It would serve no purpose if I were to try to describe most of them to you because however fast your eye is, the monte tosser's hand is faster.

However one trick probably needs to be listed here because it is so often the coup de grace that finishes the "game" and the sucker's bankroll.

When the operators of the game think the sucker has lost enough to be eager to recoup, one of the outside men creates a brief distraction. He may appear to fumble and drop some money, or one of the cards.Or perhaps he will pretend to start an argument with the monte tosser. When the tosser stoops to pick up what has been dropped, or appears to be preoccupied with the argument, a second outside man "crimps" (bends a corner of) the queen of spades.

When the game is resumed, the outside man who is playing the role of cheater puts a large wager on the bent card, and wins. The sucker can hardly wait for the next toss to bet everyting he has on the sure thing.

But when the card is turned up, it is a red ace with a bent corner, and the queen has miraculously straightened itself out. The victim, flat broke, is in the mood to jump into the river.

However, the monte mob departs with no sense of guilt. Con men just shrug and say, "Suckers have no business with money, anyhow."

In John Scarne's "Complete Guide to Gambling," the man who is billed as "the world's foremost authority on gambling" says that Three Card Monte is closely related to (and probably derived from) the old shell game, which is also known as the thimble game. In these "games," a pea or an ersatz pea made of rubber or other material is shifted rapidly among three walnut shells, thimbles, or other covers.

The sucker is challenged to pick the shell under which the pea rests, but he has zero chance of picking the right one unless the con men want him to.

The pea is not under the shell he bets on because it isn't under any shell. It is hidden in the "thimble-rigger's" hand. If you think you're too smart to be fooled in this manner, rember that thousands of magicians, including amateurs, don't have to work with a tiny pea to fool you. They do the trick with a sponge rubber ball an inch or two in diameter. You see the ball under one of three silver bells, but when the magician lifts the bell again a moment later, the ball has vanished.

Scarne calls "Cups and Balls" the world's oldest conjuring trick and says it was known in the Second Century.

Don't aske me how these things are done. I don't know how. It is sufficient to know that they are done.

Scarne says we positively cannot follow the sleight-of-hand of a good operator, and he warns us not to bet against one.

He says there is only one way to avoid being cheated: When you see the game being played, turn around and walk away. Quickly.