the world is not only hungry,but also thirsty for water. this may seem strange to you, since nearly 75 % of the earth's surface is covered with water. but about 97% of this huge amount is sea water, or salt water. man can only drink and use the other 3% -- the fresh water that comes from rivers, lakes,underground, and other sources. and we cannot even use all of that, because some of it is in the form of icebergs and glaciers. 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. let me give you just one small exampie of how necessary water is to industry. did you know that to produce a single ton of steel, it takes about 91, 000 liters of water?
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 on a small scale. for us, the systems are as important as the spacecraft.
节约用水
我们的世界不仅饥饿，而且因为缺水而干渴。对你来说这可能似乎奇怪：因为我们的地球表面75％由水覆盖着。但是，这巨大的数量中97％是海水、或咸水。只有剩下3％
的来自河流、湖泊、地下，和其他来源的淡水供人们饮用、使用。即使如此，这3％的水源我们也不能够完全使用，因为有些淡水是以冰山或冰河的形式存在。更糟糕的是，一些水源已遭受了污染。
然而，尽管按现状看来，这些少量的淡水即使时常转化成雨水的形式，对我们来说，也仍然够用。可是我们对水的需求却日益迅速增长。我们只有现在采取措施解决这一问
题才能避免未来世界范围内的严重水资源缺乏。限量供水会对农业和工业产生不良的影响。我们举个小小的事例就能证明水对工业的必要性。每生产一吨钢材就要消耗91，000升水，你知道吗?
我们必须杜绝对宝贵水资源的浪费。我们应该采取的第一步就是要发展水再利用的方法。该领域的实验已在进行，但是仅是小规模地进行。水资源再利用系统的重要性
对我们而言，与空间站不相上下。