The Reason Why You Can't Photocopy Paper Currency

People always wonder about whether defacing, copying or destroying money is against the law, questions like- How against the law is it? or Why can't I make some fake money with a photocopier for my movie?
These questions are on the tip of our tongues when currency laws come up in conversation, but people aren't asking the one question we're all curious about- what mysterious force stops us from photocopying money?
You may have heard about the Eurion constellation effect on older bills, but today's bills have a new defense system built in which makes it virtually impossible to copy- the Counterfeit Deterrence System.

(YouTube Link)

This interesting video by Wendover Productions explores the mysterious Counterfeit Deterrence System which stops photocopiers and image editing software like Photoshop from printing phony bills. So much for my retirement plan!