http://en.wikipedia.org/wiki/Legal_tender
Legal tender is a medium of payment allowed by law or recognized by a legal system to be valid for meeting a financial obligation.[1] Paper currency is a common form of legal tender in many countries.

For instance, if you owe someone $100 USD in the United States, you can try to pay it back in Mexican Pesos or valuable jewelry or gold metal, or even a cheque or a charge card, but they don't have to accept any of those as payment. They must accept US $20 dollar bills, however, because that is legal tender in the United States.
Read that last line over and over again until you realize that a $20 bill is legal tender IN THE UNITED STATES. Now what is the UNITED STATES? It is a municipality aka THE DISTRICT OF COLUMBIA. Now what is a MUNICIPALITY? According to Bouvier:

MUNICIPALITY. The body of officers, taken collectively, belonging to a city, who are appointed to manage its affairs and defend its interests.
THE DISTRICT OF COLUMBIA (AKA THE UNITED STATES) IS NOT FOUND ON A MAP ... IT IS A BUNCH OF PEOPLE ACTING AS OFFICERS. Chief among these people is one de facto president known as Barack Obama.

Are YOU in this body of officers? That depends upon whether you claim to be a citizen or not. If you do then you MUST accept that $20 bill as legal tender. If you are not a citizen you accept it under your own discretion (and frankly you would be better off asking if there is any better security).