You are sure to be familiar with wind power plants: the high towers that are sometimes to be seen in the countryside. On the top there are vanes that turn when they are driven by the wind. These power plants are put up in places where it is mostly very windy, such as on the coast. They use the wind as energy, because it is always renewed. In contrast with oil or coal, which are burned in power plants to produce electricity, wind will never be exhausted. The sun, the internal heat of the earth and the movement of water can also be used to win energy. The advantage, or to put it better, the good thing, is that nothing is used up - just used. Renewable energy sources also include raw materials that grow again, such as wood. But renewable energy sources also have to be treated with care. For example, only a certain number of trees should be cut down, and newly planted trees must be given enough time to grow again. Some sorts of tree, such as oaks, have to be managed particularly carefully, as they grow extremely slowly.