4. Passage Nine Today anyone will accept money in exchange for goods and services. People use money to buy food， furniture， books， bicycles and hundreds of others they need or want. When they work， they usually get paid in money. Most of the money today is made of metal or paper. But people used to use all kinds of things as money. One of the first kinds of money was shells. Shells were not the only things used as money. In China， cloth and knives were used. In the Philippine Islands， rice was used as money. In some parts of Africa， cattle were one of the earliest kinds of money. Other animals were used as money， too. The first metal coins were made in China. They were round and had a square hole in the center. People strung them together and carried them from place to place. Different countries have used different metals and designs for their money. The first coins in England were made of tin. Sweden and Russia used copper to make their money. Later， other countries began to make coins of gold and silver. But even gold and silver were inconvenient if you had to buy something expensive. Again， the Chinese thought of a way to improve money. They began to use paper money. The first paper money looked more like a note from one person to another than paper money used today. Money has had an interesting history from the days of shell money until today. Why does the author say that even gold and silver were inconvenient if you had to buy something expensive?