1. 人们以为淡水是取之不尽的 (提示：雨水、河水、井水......)
2. 实际上淡水是非常紧缺的 (提示：人口增加，工业用水增加，污染......)
3. 我们应该怎么办 Global Shortage of Fresh Water
范文：
Many people believe that the world’s supply of fresh water will never be used up. They think that fresh water comes from such a variety of sources that it is always sufficient for use. Such an opinion leads to a shocking waste of water.
As a matter of fact, the earth is short of fresh water. Furthermore, with the rapid growth of the world’s population, the rising demand for water by industry, and the serious pollution of our surroundings, the world is facing the danger of running out of fresh water. Actually, in some big cities, fresh water cannot meet the daily needs.
Therefore, to find new ways to save water is an urgent task. First, we should pass strict laws to control any waste of water. Second, scientists must work even harder to purify sea water and polluted water, and try to find substitutes to reduce the use of fresh water.
