Here's why:

* Limestone is primarily composed of calcium carbonate (CaCO3).

* Metamorphism is the process of changing existing rocks (in this case, limestone) by heat and pressure, without melting.

* Marble is a metamorphic rock formed from limestone. The heat and pressure during metamorphism cause the calcium carbonate crystals in the limestone to recrystallize and interlock, resulting in a denser, harder, and often more colorful rock.

So, when limestone is subjected to the right conditions of heat and pressure, it transforms into marble.