system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do beaches have sand? Where does the sand come from and why is it all in between the ocean and the main-land?
gpt
Water constantly slamming into land causes rocks to break down into tiny particles. What you get is a lot of sand. As the water continues to slam into the shore it pulls the sand out with it into the ocean slowly destroying the land around it. This is why beach front property is a risk. You might end up with a house in the water a few years down the road. 

To combat this many places actually bring in tons of sand to add to the beach to replace what is taken by waves.