Oil is so important that it is sometimes called "black gold". Almost half of our energy comes from oil. We use it to run our cars and factories and to heat our homes, offices and schools. Many everyday things are made from oil. Your shirt may have oil in the material. The soap you wash your hands with might also be made from oil. Your favorite plastic toy is made from oil. Oil is hard to find, because it is trapped deep under the earth. Once the only way people knew there was oil someplace if it leaked out of the ground. Today, however, we have many ways of finding oil. One tool measures the pull of gravity. Places where gravity is weaker are more likely to have oil. Another tool is sound waves . Sound waves travel through different kinds of rocks at different speeds. We can use them to find the rocks that have oil in them. We need a lot of oil, and we are using up the oil wells we know about. Soon we must find new ways of looking for this "black gold". The main idea of the whole story is that   _  .
many things are made from oil we must find new ways of looking for oil people spend a lot of time looking for oil oil is important for us but hard to find