system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there sand at the beach?
gpt
It's caused by geological processes called mechanical weathering, transporting, and sorting. The previously formed rocks in the area are broken down by natural forces. I.e. water currents/waves, wind, ice expansion/melting, and tree roots breaking through rocks. These broken rock fragments or sediment deposits are then moved during a process called transportation. If it's a beach, then most likely the transport medium will be waves and ocean currents. 

The sand is then sorted near the shore by how much force is involved in the transplant. Ex. Larger rocks deposited further from shore because the wave loses energy as it gets closer to shore and cannot move heavy objects. The energy in the waves, by the time it reaches shore, is just enough to move small grains. Usually beaches will have mostly the same sized grains make up the beach.

Notice that each beach has different sized sand grains. Some beaches have very fine sand. And some have very gritty large-grained sand. This is all due to the energy of the waves that reach the shore. The wave size is affected by the slope of the beach. Steep slopes lose energy faster, while gradual slopes lose energy slower. This is why Hawaii has huge waves. Some beaches have black sand. This is usually from volcanic activity near the beach that happens under water. The waves bring in whatever materials that it can push.

Hope this helps.