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 ..