Here's why:

* Calcite: Limestone is primarily composed of the mineral calcite (calcium carbonate).

* Evaporated seawater: Calcite can precipitate directly from seawater as it evaporates, forming layers of limestone.

* Animal shells: Many marine organisms (like corals, mollusks, and foraminifera) have shells made of calcium carbonate. When these organisms die, their shells accumulate on the ocean floor, eventually forming limestone.

Therefore, limestone is a sedimentary rock that forms from the accumulation of calcite, which can come from both chemical processes (evaporation) and biological processes (animal shells).