Unfortunately, no one can demand gold or silver in exchange for their paper money, or notes as they are called, from the American Government anymore. Our currency, like many other nations, is known as flat currency. Basically, this means the dollar you hold is how much it is worth, no more, and no less.
n 1963, an act was passed to remove the words, "Will pay to the bearer on demand," from all money made from then on out. This prevented the American people from turning in their money for the gold or silver it was worth. Nowadays, even if you happen to come across a note from before the year 1963, you cannot redeem it for gold.
The Government will only exchange the old money for new money in the same amount. But, to some collectors, paper money from before the year 1963 is worth more, and some will even pay a decent sum for older currency, depending on the collector.