How does rock become sand on a beach?

Answer: Weathering and abrasion break rock into grains that water can transport

Seawater dissolves rock and leaves the dissolved material as sand — Material dissolved in seawater is not itself a collection of sand grains. Rock-derived sand consists of solid fragments that survive weathering and abrasion. Shells and coral can also contribute solid particles when they break down.

Weathering and abrasion break rock into grains that water can transport ✓ — Weathering breaks rock apart, and abrasion wears the fragments into smaller grains. Rivers, waves and currents can transport and sort them onto beaches. Not all beach sand comes from rock: some beaches contain abundant shell or coral fragments.

Wind turns intact rock directly into sand as it carries it to the coast — Wind can move grains that already exist, but it does not turn intact rock straight into airborne sand. Weathering and abrasion first produce the particles. Grain shape alone does not reliably tell you whether a grain came from a desert or a beach.