system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there sand on the coast?

The question might be a bit too vague, so to elaborate... Why is sand naturally present where the ocean and land meet? Why are there no (or not commonly) dirt/mud beaches. How does the sand get there and stay there?
gpt
Dirt comes from decayed plants. Sand comes from broken-up rocks. 

When plants die, new ones usually grow. So places where dirt is next to ocean are those places where plants grow right up to the edge, instead of having a wide-open "beach" area. 

Plants don't grow so well on sand, so it's easier for beaches to "stay" beaches.

EDIT: Sand gets eroded from rocks and carried by rivers down into the ocean. Then ocean currents and waves carry the sand up onto the land and flow back down without the sand, leaving it deposited. Kinda like how whales or fish can get beached on land if they get into water that's too shallow; sand gets deposited the same way.