The world is not hungry, but it is thirsty. It seems strange that nearly 3/4 of the earth is covered with water while we say we are short of   water. Why? Because about 97% of water on the earth is sea water which we can't drink or use for watering plants directly. Man can only drink and use the 3%--- the water that comes from rivers and lakes. And we can't even use all of that, because some of it has been polluted. Now more water is needed. The problem is: Can we avoid a serious water shortage   later on? First we should all learn how to save water. Secondly, we should find out the ways to _ it. Scientists have always been making studies in the field. Today, in most large cities water is used only once and then runs to the sea or rivers. But it can be used again. Even if every large city reused its water, still there would not be enough. What could people turn to next? The sea seems to have the best answer. There is a lot of water in the sea. All that needs to be done is to get the salt out of the sea water. This is expensive, but it's already used in many parts of the world. Scientists are trying to find a cheaper way of doing it. So you see, if we can find a way out, we'll be in no danger of drying up. ,,. From the passage we learn   _  .
if every city reused its water, we'll be in no danger of drying up man can only drink and use about 25% of water on the earth today in most large cities water is used only once water can be used only once