To quote Ben Bernanke 11/21/02

…"Like gold, U.S. dollars have value only to the extent that they are strictly limited in supply.

But the U.S. government has a technology, called a printing press (or, today, its electronic equivalent), that allows it to produce as many U.S. dollars as it wishes at essentially no cost.

By increasing the number of U.S. dollars in circulation, or even by credibly threatening to do so, the U.S. government can also reduce the value of a dollar in terms of goods and services, which is equivalent to raising the prices in dollars of those goods and services.

We conclude that, under a paper-money system, a determined government can always generate higher spending and hence positive inflation."