I found this part to be quite surprising.
The Basics: How much is a dollar worth?
The question seems simple, but the answer is quite complex. Since 1973, the dollar bill has had no value tied to it. You cannot trade in a dollar to the government for gold, silver, or any other commodity. The value of the nation's currency is related to the decree by the government that a dollar is legal tender for all debts. This means if someone attempts to pay a debt using dollars, the person being paid must accept the money or the law no longer recognizes the debt. This is important enough that the phrase is printed on every bill the government creates.
15 Things You Never Noticed on a Dollar - Manage Your Life on Shine