75% of the earth’s surface is covered 46 water. However, the world is not only hungry but also thirsty for water. It seems 47 to you, isn’t it? In fact, about 97% of water on the earth is sea water 48 salt water, which can’t be used directly. Man can 49 drink or use the other 3 % of the fresh water 50 comes from rivers, lakes and underground. Some of it is in the form of icebergs (冰山) and glaciers( 冰川 ). As things stand today, this small amount of fresh water is still enough for us. On the other hand, because 51 number of people in the world 52 larger and larger, more and more fresh water is needed. That’s 53 water shortage becomes a big problem. How can we 54 this problem? It’s both the scientists’ and 55 duty to find some ways to turn salt water into fresh water.