Rocks are broken down by physical changes in mechanical weathering. One type of physical change is frost action. This is when repeated freezing and thawing of water breaks rocks apart. Water seeps into cracks, and as the temperature drops, it freezes. The ice expands, widening the cracks. Then more water seeps in, repeating the process. Another physical change is the growth of plant roots in rock cracks. Moles, earthworms, and other animals that burrow turn over soil. As a result, these rocks are exposed to further weathering processes. Moving water is another form of mechanical weathering. As water moves, it can transport pieces of broken rock. These rocks can be broken down into smaller rocks through rapids or crashes against the shore.