The earth is always changing. One way it changes is by erosion. Some is caused by the weather. For example, the wind causes erosion. In a desert, the wind blows the sand around. Rain also causes erosion. It washes away earth and even changes 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 lower and the land is flatter.