"The only difference between men and boys is the cost of their toys."

- Mechanical-bank owner, age 41.

Grown men and women lie about them, fight over them, and - sometimes - feed them coins to see them move. They're cast-iron mechanical banks, glorified piggybanks with action, now coveted as antiques. They depict Bible stories, animals, childhood scenes or comments on the politics of their day.

In the late 1800s and early 1900s, when these ingenious toys were made, they painlessly taught children to save money. For a penny, a child with a circus bank got a show: a clown in a pony cart driving around the ring, raising his hand and knocking the coin into a box.

The banks reflected their times, depicting attitudes considered objectionable today. Some showed a hapless black man being butted by a goat, buffalo or mule, or thumbing his nose after making a deposit in the "Freeman's Bank." Chinese and Irish stereotypes were also common.

The circus bank, which cost about 70 cents when first made in 1889, is now worth $2,000 to $4,000, depending on how well it still works and looks; prices of other mechanical banks range from $125 to $20,000 or more. In all, about 300 types of banks were patented before 1935. They were produced in varying quantities that now determine availability and, hence, price.

Take the case of the tiny iron turtle.

Around the turn of the century, Kilgore manufacturers in Westerville, Ohio, made what was then a very popular bank, a rabbit whose ears perk up when you feed him a coin. Kilgore made just a few of a similar but less-loved bank, a turtle; when you put a coin in his back, he nods his head. Both sold for 35 cents, but today the rabbit is worth only about $125, while the turtle commands around $15,000.

The truly avid collector wants one of each cast-iron bank ever made. One way to this end - if you own five banks and can get a member to sponsor you - is to join the Mechanical Bank Collectors of America and, at the annual reunion, swap banks and bank lore. If you're not in the club, you may be able to buy a mechanical bank from auctioneers such as Weschler's and Sloan's in Washington, and Thieves' Market in Alexandria; through collectors' classified ads in hobby publications and antique journals, or from a dealer.

Friday and Saturday the annual eastern national antique dolls, toys and games show and sale will be on at the Gaithersburg fairgrounds, and there are likely to be 50 to 60 different mechanical banks for sale by three or four dealers.

One nearby dealer is Frank Whitson in Baltimore. His shop, Antique Mania, 229-231 West Reed St., usually has from 50 to 100 old banks on hand.

Beginning bank collectors should start off slowly and beware the bugaboo of antique-seekers - reproductions.

In 1940 the Book of Knowledge copied some old banks and embossed these words on the base: "reproduced from original in collection of The Book of Knowledge."

Unscrupulous dealers have been known to file the words off. But the Book of Knowledge reproduction is further identifiable by an indented circle on the base, which can't be filed off. Though "Made in 1860" is written on it, the bank's worth about $25.

Some recent fakes can be spotted by their texture or size.

Back when the originals were made, foundries used very fine sand in casting, and the result was a smooth product. Craftsmen made the cast-iron bank even smoother by tumbling it over and over in a machine. Reproductions made now may be rough inside and out, as if iron casting were a lost art. It's a simple cast of not making 'em like they used to.

Fakes made by using a new mold made from an old bank can be detected by their size: Iron shrinks as it cools, so the copy will always be smaller than the original.One enterprising purist, Robert McCumber, has published a book of base tracings. If you have any doubts about authenticity, you can put the base of your bank down on a tracing of the base of an original. If your bank's smaller, you've probably bought an imitation.

Another way to tell some fakes is by their metal: Today's cast iron is generally heavier, while if brass, pot metal or lead is substitued, a rough, pebbly quality is apparent. If you're still in doubt, a magnet will tell you if a bank is iron.

Often you can tell a fake by its paint. Collectors just don't repaint old mechanical banks. They're more valuable with the original paint, even if - as is usually the case - it's scratched and rubbed off. Modern synthetic paint - any garish color - is a tip-off. Naturally, someone's thought of painting an imitation to make it look old. It's done with flat paint that resembles old oil paint.

Other ways to make a new bank seem old include dipping it in salt water or acid or burying it in the ground or in cow manure, to "age" it. Your best protection against this sort of fraud is simply checking the reputation of the people you're dealing with.