Nearly all our food comes from the soil. Some of us eat meat, of course, but animals live on plants. If there were no plants, we would have no animals and no meat. So the soil is very necessary for life. The top of the ground is usually covered with grass or other plants. There may be dead leaves and dead plants on the grass. The waste matter(粪) from animals also falls on it.

    All soil needs food. If we do not give it any, the plants will be weak. Farmers found that animals' waste is the best food for the soil, but chemical fertilizers(化肥) are also very useful. The same crop（庄稼）should not be grown in the same place every year; it 's better to have a different crop. A change of crop and the use of a good fertilizer will keep the land good.

    Trees are useful to man in three important ways: they give him wood and other useful things, they give him cool places, and they help to stop drought and flood.

    Unluckily, in many parts of the world, man has not found that the third of these points is the most important. Man wants to make money from trees, so he has cut them down in large numbers, only to find that without them he has lost the best friends he had. And also, he is usually too careless to plant and look after new trees. So the forests slowly disappear.

    This does not only mean that man will have fewer trees. The results are even worse: for where there are trees, their roots break up soil make the rain in——and also bind the soil, thus stopping it from being washed away easily; but where there are no trees, the rain falls on hard ground and flows away, causing floods and carrying away the rich top-oil. When all the top soil is gone, nothing is left but useless desert.

    A myth is something that is not correct, but many people believe. There are a lot of myths about the human brain.

    One of the biggest myths is that we only use 10 percent of our brains. The next part of the myth is that if we can learn to use the rest of our brains, then we'll be much smarter. People say this all the time, but it's absolutely not true! The truth is that although we don't know everything about the human brain, we know that each part of it has an important function (功能). Modern scientists think the "10 percent myth" ridiculous(荒谬的).

    The other most popular myth is about being "right brained" or "left brained". According to this myth, people who use the right side of their brains are more artistic and creative. People who use the left side of their brains are better at math and science. This is as popular as the 10 percent myth, and it's also wrong. In 2013, a study at an American university examined the right brain and left brain myth. According to the study, we use both sides of our brains equally (相等地).

    It's true that we use different parts of our brains for different things. We use our left side for language more, and our right side when we need to pay attention. But there is no evidence (证据) that creative people use the right side more, or that scientific people use the left side more.

    We've talked about myths, so let's look at a few interesting facts about the brain. First of all, the brain feels no pain. Second, about 75 percent of the brain is made of water. It's also the fattest organ in your body. Here's another interesting fact about the brain. Around the time you turn 18 years old, it stops growing.