The earth is always changing. One way it changes is by erosion Some erosion is caused by the weather. For example, the wind causes erosion. In a desert. the wind blows the sand around. Rain 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.