【答案解析】Money is usually defined as medium of exchange. That is, you receive money in exchange for the work you do. In turn, you give it to people in exchange for the things you want, such as food, clothing, or other consumer goods. Many different things have been used as money. In some places, seashells have served as a medium of exchange. However, gold, silver, and copper have been the most popular kinds of money throughout the ages. Coins are still made of silver and copper in most places today. However, most of the world's money is made of paper, which has almost no value in itself.