The world is not only hungry；it is also thirsty for water.This may seem strange to you because nearly 75%of the earth’s surface is covered with water.But about 97%of this huge amount is seawater or salt water.Man can only drink and use the other 3%—the freshwater that comes from rivers，lakes，underground，and other sources.And we cannot use all of that，because some of it is in the form.of iceberg and glaciers （冰河）.Even worse，some of it has been polluted.
However，as things stand today，this small amount of fresh water，which is constantly being replaced by rainfall，is still enough for us.But our need for water is increasing rapidly almost day by day.Only if we take steps to deal with this problem now can we avoid a severe worldwide water shortage later on.A limited water supply would have a bad effect on agriculture and industry.
We all have to learn how to stop wasting our precious water.One of the first steps we should take is to develop ways of reusing it.Experiments have already been done in this field，but only a small scale.The systems that have been worked out resemble those used in spacecraft.

The first sentence ’The world is not only hungry；it is also thirsty for water ’means’（）’.