How are bills made to discourage counterfeiting?

How are bills made to discourage counterfeiting?
The United States Mint includes quite a few security features in United States currency in order to discourage counterfeiting (and make it easier to detect). Several off these include the following:

U.S. currency is printed not on paper, but rather on cotton. The 'paper' on which it is printed has a content of approximately 75% cotton and 25% paper. This gives the bills distinctive texture and durability.

Bills are printed using printing plates, an old technology, but one that permits extremely fine detail, which makes a number of other features and fine textures possible. In addition to extremely fine details that are nearly invisible to the naked eye, all U.S. bills contain microprinting in strategic locations, which are difficult to discern from fine lines. Look at a bill under a magnifying glass or jeweler's loup at 20x or 30x magnification, and they will be easy to see. Examples include the words, "United States Fifty Dollars 50 USA 50" detail and border elements on new bills stating the denomination, which can be easily mistaken for decorative borders, or the shadows on the wavy 'Twenty USA' which under magnification reveals the same words in microprint.

U.S. currency also contains watermarks, for example Abraham Lincoln's face beneath the oversized $5, and the American flag detail on some bills.

Tiny red and blue fiber flecks are present in the paper of all bills; this is one of the older security methods employed. Newer bills also incoporate silver and gold raised 'puffy paint' on indicia in the lower corner.

Vertical metallic ribbons with the stated denomination are employed in U.S. bills as both a security measure and as a check against large amounts of cash being moved internationally (large amounts of bills will set off metal detectors and can be spotted through X-ray analysis at airports). When the bills are discovered upon search, the proper authorities can be notified and criminal activity uncovered. These bills also have the potential to permit remote counting using these ribbons.

All denominations (besides the $1 bill) incorporate multi-color security printing to make the bills more recognizable and difficult to falsify. This is a very inexpensive method that was adopted by most foreign nations long before the U.S. put it into practice. In fact, now the entire background is printed in a dynamic color pattern, which is nearly imperceptible, except under magnification. Also, these low-contrast patterns are photographically neutral -- the human eye discerns the color differences as quite stark, but scanning or photographing them destroys their contrast or the clarity of the printing itself, making a counterfeit easy to spot.

The extra-large denomination in the corners was adopted to deter counterfeiters from modifying the denominations on bills to increase the face value... for example doctoring a $5 bill into a $50 bill. The recognizable giant '5' makes this nearly impossible.

U.S. bills are also printed with invisible and ultra-violet ink with patterns that are only visible under special lighting.

And, of course, serial numbers are validity-coded, much like credit card numbers, and so cannot be simply invented. The series numbers, and alphanumeric combinations are unique, and (obviously) different on each bill. Thus, this helps the bills to defeat any counterfeiting method that relies on simply photocopying bills.

In short, the job of a Secret Service agent must be as exciting as a Maytag repair man... U.S. money is that secure these days.