The earth is always changing. One way it changes is by erosion. Some is caused by the weather. For example, the wind cause erosion. In a desert, the wind blows the sand around. Rain also causes erosion, it washes away earth and even changer the shape of some rocks. Another kind of erosion is caused by rivers. When a river goes through a mountain, it cuts into the mountain. After a long time, the mountain is tower and the land is flatter.