For as long as there has been money, there've been people trying to make fake money. In the U.S., the latest effort to stop counterfeiting will arrive October 8, when the next $100 bill is released.
The Federal Reserve has mixed in new features to make the bill more difficult to replicate, like a blue security ribbon running down the front. When the bill is tilted, columns filled with the number "100" and tiny bells rotate. Tilt left to right and the columns move up and down; tilt up and down, the columns move left to right. The ribbon is woven onto the paper, not printed, making it tougher to duplicate. It's composed of thousands of lenses that magnify the marks and make them appear to move in different directions--not easy for a counterfeiter to remake in a basement.
To the right of Ben Franklin is an ink well with a bell inside, and both the well and bell look copper-colored until someone turns the bill, which changes the bell to green. The "100" printed in the bottom right changes color the same way. That's probably made from optically variable ink, which is used for anti-counterfeiting because it's difficult to obtain (very few companies sell it) and can't be color-copied to change in light.