The world is not only hungry, but it is also thirsty for water. This may seem strange to you , since nearly 70% of the earth's surface  is covered with water . But about 97% of that is sea water, or salt water. Man can only drink and use the other 3% -- the fresh water from rivers , lakes , underground , and other sources . And we can not even use all of that. Even worse, some of it has been made dirty.     However, as things stand today, this fresh water is still enough for us . But our need for water is becoming larger and larger quickly -- almost day by day . We should take steps to do with this problem now and in this way we can get away from a serious world-wide water shortage later on .     We all have to learn how to stop wasting our valuable water. One of the first steps is to develop ways of reusing  it .Today in most large cities , water is used only once and then sent out into a sewer system . From there it returns to sea or runs into underground places.    But even if every large city reused its water, we still would not have enough. All we'd have to do to make use of sea water in the world is to remove  the salt . If we take these steps we'll be in no danger of drying up. The best but the most difficult way to solve the water problem of the world is to   _   .
remove the salt from sea water make the water in cities clean take water from underground make the need for water smaller