Modem zoos are different from those built fifty years ago.At that time zoos were places where people could go to see animals from many parts of the world.The animals lived in cages with iron bars（栅栏）.Although the zoo keepers took good care of them,many of the animals did not feel comfortable,and they often fell ill.

    In modern zoos,people can see animals in more natural conditions.The animals are given more freedom in larger places so that they can live as they would in nature.Even the appearance of zoos has changed.Trees and grass grow in cages,and water flows through the places the animals live in.There are few bars;instead,there is often a deep ditch（沟）,filled with water,which surrounds a space where several sorts of animals live together as they would naturally.In an American zoo,the visitors can walk through a special huge cage that is filled with trees,some small animals and many birds,and large enough for the birds to live naturally.In a zoo in New York,with the use of special nights,people can observe certain animals that are active only at night,when most zoos are dosed.Some zoos have special places for visitors to watch animals that live in the desert or under water.

    Modern zoos not only show animals to visitors,but also keep and save rare animals.For this reason,fifty years from now,the grandchildren of today's visitors will still be able to enjoy watching these animals.

注：图中一种小球代表一种元素的原子， 分别代表C、N、O三种原子．