Science - Why is there sand at a beach?

We are learning about the earth and how it has changed over time. Yesterday we had fun doing a science lesson about why there is sand at a beach. We learned that rocks and big boulders often fall from higher elevations into a rushing river and the big rocks get split in half, then in half again and so on and so on until they are small pebbles or sand grains.  Students demonstrated this process by cutting paper "boulders" in half, then in half again and again so they could see the visual of this process. We had a lot of fun!