OPEN UP your wallet, and take a look at some of those green pieces of paper you carry around.

Money is strange stuff, as strange as any invention of science fiction. A few writers have considered the peculiarities of this strange invention—from Cory Doctorow's exploration of the concept of reputation-based economics in Down and Out in the Magic Kingdom to Neal Stephenson's Baroque cycle, which deals with the nature of money and includes Isaac Newton as a character.

In the Stephenson series, the King of France sends a counterfeiter to England to destabilize the currency, and England brings in one of the most intelligent persons of the age, Isaac Newton, to combat the threat. Back in Newton's time, "clippers" slyly clipped valuable metal off the edges of coins and then spent the coin at its full value. As master of the mint (yes, he really was), Newton thwarted clippers by milling the edges of coins. If the milling was gone, you knew someone had been snipping off bits of the coin.

Today people don't clip the edges of dollar bills, since the paper of the bill has little intrinsic value. But counterfeiters still come up with clever ways to produce bogus money. On the day that we are writing this column (September 20, 2007), the U.S. Treasury is announcing a new design for the five-dollar bill, one that includes a number of new anti-counterfeiting features.

In this column, we're going to examine those pieces of paper from your wallet and we're going to consider the latest skirmishes in a technological war has been going on for centuries—the ongoing battle between the legitimate minters of money and the counterfeiters. The U.S. Bureau of Engraving and Printing strives to make currency that's durable, relatively cheap to manufacture, and impossible for anyone else to duplicate. And criminals who want to make a quick buck strive to fabricate convincing counterfeits, pieces of paper that look like the ones the Bureau produces. This battle has led to some significant changes in money you use—and will lead to more changes in the future.

A TWISTED HISTORY

Counterfeiting has a long and illustrious history, with some great twists and turns.

Back in the mid-1800s, for instance, a clever counterfeiter discovered how to make counterfeit gold sovereigns that had the correct density. To accomplish this, the counterfeiter alloyed platinum (which was then cheaper than gold) with copper to make coins with the correct density but less gold than the real thing. Of course, with the current value of platinum twice that of gold, those fakes are likely to be worth more than the originals today.

Counterfeiting has long been an international business. When the American colonies started printing paper money, copies of that currency were printed in Germany, England, Amsterdam, and Ireland, where manufacturing this bogus American currency wasn't illegal. During the American Revolution, the British counterfeited the dissident colonies' currency in an effort to undermine the new government.

The U.S. didn't have a national currency until 1863. Before that time, each bank issued its own currency. Since each bank had its own designs, it's no wonder people found it tough to identify counterfeits among the 7,000 different varieties of legitimate bills. Estimates indicate that one third of the currency in circulation in the early 1860s was bogus. There was a thriving business in "Counterfeit Detectors," monthly lists describing the bogus bills and how to identify them.

When a national currency was adopted, counterfeiters didn't hesitate to duplicate the new "greenbacks." Pat has a favorite counterfeiter: Thomas Peter McCartney, who lectured as Professor Joseph Woods during the period from 1863 to 1868. The learned professor lectured on the art of detecting counterfeits. His talks were illustrated with the nineteenth-century equivalent of a Power Point presentation: large and detailed drawings on which he pointed out defects of many of the better-known counterfeits.

Of course, right after Professor Woods left town, some of Thomas Peter McCartney's confederates would arrive and pass some excellent counterfeits that lacked the flaws that the knowledgeable professor had described. Apparently lecturing alone was not lucrative enough for the good professor.

JUST PAPER AND INK

Are you still clutching those bills you pulled from your wallet? Before we get into the high-tech innovations that have been introduced in the last twenty years, let's take a look at some of the safeguards against counterfeiting that date back to 1863.

First, examine the paper on which the currency is printed. This paper is produced by Crane & Company of Massachusetts and shipped to Washington, D.C., in armored trucks. Possession of this paper by unauthorized individuals is a federal crime.

Currency paper is made with a special blend of linen and cotton fibers, with three pounds of red and blue fibers for every ten thousand pounds of untinted fiber. If you look at a bill under a bright light with a magnifying lens, you may be able to spot some red and blue fibers in the paper. Those fibers make the paper difficult to duplicate. To be convincing, a counterfeit bill must contain those tiny streaks of red and blue. Some counterfeiters have drawn or printed red and blue lines on their bills; others have glued tiny fibers on.

There are other ways to distinguish currency paper from regular paper. Under an ultraviolet or "black light" lamp, ordinary paper will fluoresce or glow as the bleaching agents in the paper absorb the ultraviolet light and reemit visible light. But currency paper won't glow, since no bleaching agents are used in its production. Paul often carries a tiny ultraviolet LED mini torch, which has an ultraviolet light emitting diode and can be used detecting counterfeit currency (among other things).

Of course, genuine currency that has gone through the washer in the pocket of your jeans will glow under ultraviolet light if your laundry detergent contains whiteners. And genuine paper is no guarantee of a genuine bill. Think for a moment and we bet you can come up with a readily available source of genuine currency paper.

That's right: a great source of currency paper is currency itself. Counterfeiters have been known to bleach a batch of dollar bills and print hundred-dollar bills on the paper. That's a very nice profit margin!

Now, run your finger over the surface of a bill. Can you feel the raised lines of ink? U.S. currency is printed on an intaglio press, which uses an engraved steel plate. The process leaves mounds of ink on the paper that you can feel. Because that ink is ferromagnetic, a strong magnet, such as a rare earth or neodymium magnet, will attract a dollar bill, a characteristic that helps both vending machines and the automatic sorting machines at Federal Reserve banks detect counterfeits.

Take a look at the elaborate scrolls and flourishes that decorate the borders of each bill. Those are also anti-counterfeiting measures. Because of the printing methods used, every curlicue and detail has crisp lines. When U.S. currency was first designed, only a skilled printer could make a convincing duplicate of a hundred-dollar bill. The printing precision required by the design was difficult for counterfeiters to duplicate—until relatively recently.

DESKTOP COUNTERFEITING

From 1939 to 1990, there weren't any big changes in the overall look of U.S. currency. Oh, there were some modifications: experiments with the composition of the paper in the forties, the addition of "In God We Trust" in 1957, the shift from Silver Certificates to Federal Reserve Notes. But the overall look and feel of the bills remained fairly static. Some other countries changed their money designs regularly, but not the United States.

In the early 1990s, that changed. Why? Because the tools available to would-be counterfeiters had improved dramatically with the development of the color copier and color printer.

In 1991, government agents recovered between six and eight million dollars' worth of counterfeit currency that had been produced using color copiers and printers. At that time, desktop counterfeiting was a growing industry, doubling each year. Design changes that the Treasury Department began introducing in the 1990s were intended to thwart these casual counterfeiters.

If you have a five or a ten or a twenty among the bills in your wallet, take a look at the series number on the bills. This number is just to the right of the portrait on the bill and it indicates the date of the design for this particular bill (not the date the bill was printed).

According to the Federal Reserve Bank of Atlanta, one-dollar, five-dollar, and ten-dollar bills generally wear out in around a year and a half. Twenty-dollar bills, which aren't handled as much, may last for two years. A fifty may be around for as long as five years and a hundred-dollar bill has an average life span of eight and a half years. So the odds are good that you'll find some bills of relatively recent vintage in your wallet.

If the bill you're examining is worth five dollars or more, look the dead president in the eye and hold the bill up to the light. Examine the blank area just to the left of the Federal Reserve seal. Unless you have a genuine antique on your hands, you'll see a clear polyester thread running vertically through the paper. The thread is printed with words that identify the bill's denomination: the twenty will have the words U.S.A. TWENTY, for example. A photocopy of the bill would lack this thread. This thread was among the first anti-counterfeiting measures included in the new currency designs.

If you have a magnifying glass, you may be able to spot another anti-counterfeiting measure: a line of microprinting that runs around the rim of the central portrait says THE UNITED STATES OF AMERICA. Back when this microprinting was added to bills, it was so small that it couldn't be reproduced with a photocopier or scanner. Those days are long past, but the microprinting remains. The microprinting and the security thread are included on all bills worth five dollars or more dating from after 1993.

Hold the bill up to the light again. Depending on the vintage of the bill, you may see a watermark, a translucent pattern created when the paper was made. Depending on the bill's denomination and age, you may see a few other anti-counterfeiting measures. The latest version of the ten-dollar bill (introduced in 2006), the twenty-dollar bill (introduced in 2003), and the fifty-dollar bill (introduced in 2004) all include numbers in color-changing ink, which looks copper colored from one angle and green from another.

One of the changes that sounds dramatic (but isn't) was the introduction of background colors other than green. The twenty-dollar bill, for example, includes peach, blue, and light green. The new fifty has background colors of blue and red and images of a waving American flag and a small metallic silver-blue star. The new ten has background colors of orange, yellow, and red. The Treasury Department's website at www.moneyfactory.gov goes on and on (and on and on) about these additional colors, but frankly, they are fairly easy to overlook.

All these security features—which can't be duplicated with a color copier or printer—have succeeded in slowing desktop counterfeiting. Something else that helped was a change in copier and printer technology: many copies and scanners and photo editing programs include currency detection features. When we tried to photocopy a twenty-dollar bill at one hundred percent (in the interest of science, of course), we got an error message. Apparently some color laser printers and copiers now encode their serial number and manufacturing code in a set of tiny yellow dots on every copy they make. We've read that these dots are visible if magnified under blue light. We looked but couldn't find any sign of these dots. The theory is that the Treasury Department can use these hidden markings to track phony money back to its source. (There are, of course, privacy issues to examine here, but that's another column.)

THE MORE THINGS CHANGE…

Back in 1996, the Bureau of Engraving and Printing announced that they would be introducing new designs every seven to ten years to stay ahead of currency counterfeiters—and they've been true to their word. The most recent new design is the five-dollar bill, which is scheduled to begin circulating in 2008. It includes a large, borderless portrait of Lincoln, a very large purple numeral 5 on the back, and background colors of purple, red, and pink.

The watermark on the earlier redesign of the five was a portrait of Lincoln. In the new bill, the watermark is a numeral 5. That change is a direct response to counterfeiting: the older five could be bleached and reprinted as a one hundred-dollar bill. By changing the watermark and the position of the security thread, the designers of the new bill want to make this form of counterfeiting more difficult.

But the more things change, the more they stay the same. Counterfeiting was an international industry back before the American Revolution—and it's an international industry today. In recent years, counterfeiting operations have been traced to Columbia, Eastern Europe, and North Korea. In Korea, a government-funded operation is alleged to produce "supernotes," almost perfect counterfeit hundreds printed on an intaglio press on paper that is identical to the official currency paper.

But counterfeits that are far less convincing than the supernotes are passed every day to people who just aren't paying attention. We think the best anti-counterfeiting measure of all is a low-tech technique that we constantly apply at the Exploratorium: take another look. Keep an eye out for the features described here. If you want to spend a little more time examining (and experimenting with) money, check out the chapter on money in Exploratopia, the Exploratorium's recent book on ways to explore everything in your world.

PROMISES, PROMISES

Money serves a variety of functions, according to economists and anthropologists. Money serves as a medium of exchange—that is, we can use it to buy stuff. It serves as a standard of value—a way of comparing the value of different things. I can buy two apples for a dollar and only one organic pluot, so money lets me compare apples and pluots. Money also acts as a store of value—a means of accumulating wealth.

Pat just can't write about money without mentioning her favorite form of money, which is used on the Micronesian island of Yap. On Yap, stone disks that measure up to twelve feet in diameter serve as a form of money. Quarried on an island that's 250 miles away and transported to Yap by boat, the ownership of these stones brings prestige to the owner. Basically, having a sizeable stone makes you wealthy.

Before you start snickering about the odd ways of the folks on Yap, think hard about those pieces of paper from your wallet. The use of paper money didn't catch on until the 1700s when goldsmiths started issuing people bills of receipt for gold they had on deposit. Those bills could be turned in and the gold delivered upon demand. People could then transfer the bills from one to another while the gold remained in the vault.

Until 1971, with a few exceptions, paper money was backed by precious metal on deposit. In 1971 the U.S. government stopped promising that it would back the value of the currency it printed with the gold on deposit at Fort Knox. Money—whatever its form—is only valuable because we say it is. That's true whether the money is big stone disks or pictures of dead presidents.

The money we use is backed by nothing more (or less) than our collective belief in it. Basically, the value of our currency is based on our faith in its value. Now that's science fiction!

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