听力题 Bacteria can make foods go bad. This is true of cooked foods as well as fresh foods. High heat kills bacteria. But warm, wet conditions help bacteria grow. So, the best way to protect food is to keep it in a cool place. There are several ways to keep food cool without the need for electricity. Some refrigerators, for example, are powered by gasoline motors. There are less costly ways than refrigeration to keep food cool. An ice box is a simple device. It is made mostly of wood. The inside walls are covered with metal. A big piece of ice is placed in the bottom. Millions of homes had ice boxes until refrigerators became common in the 1940s. An ice box will keep food from spoiling for several days if the outside temperature is not too warm. If the box is kept out of sunlight, the ice will last longer. Probably the best method to keep food cool without electricity is a device called the evaporative cooler. An evaporative cooler is easy to make. And it does not need ice. One common evaporative cooler is built as a tall box. Food is kept on several shelves inside. The shelves are made from pieces of metal with many small holes. All four sides of the box are covered with thick cloth. A container of water is placed at the bottom. Another container of water is placed at the top. The top and bottom of each piece of cloth is placed in the water. This keeps the cloth wet. Put the cooler out in the open air, but not in the sun. Air will pass through the wet cloth. This keeps the inside of the box several degrees cooler than the outside air temperature.