But, that's not the whole story! Here's a deeper explanation:

* The source: The sand that forms sandstone comes from the weathering and erosion of other rocks, like granite, limestone, or even older sandstone.

* The process: Over time, these sand grains get transported by wind, water, or ice. They accumulate in layers, and with pressure and cementation, they eventually become hardened into sandstone.

* The cement: The sand grains are held together by a natural cement, often made of minerals like calcite, silica, or iron oxide.

So, while the answer is "sand", the real story involves a complex process of erosion, transportation, and cementation of rock fragments, ultimately forming a new rock: sandstone!