Here's why:

* Small sea animals: Many types of small sea creatures, like foraminifera, clams, snails, and corals, have hard shells made of calcium carbonate.

* Pressed together: Over millions of years, the shells of these creatures accumulate on the ocean floor. The weight of the water and sediment above compresses these shells, along with other organic materials, into a solid rock.

* Limestone: This resulting rock is called limestone. It's primarily composed of calcium carbonate and is a common sedimentary rock.

Let me know if you have any other questions!