Here's why:

* Limestone is a sedimentary rock: It forms from the accumulation of marine organism remains like shells and coral.

* Mountains can be formed from various types of rocks: They can be formed from igneous rocks (like granite), metamorphic rocks (like marble, which can be formed from limestone), or even sedimentary rocks like sandstone.

Examples of limestone mountains:

* The Appalachian Mountains in the eastern United States have significant limestone formations.

* The Alps in Europe have limestone peaks and valleys.

Examples of mountains not made of limestone:

* The Himalayas are primarily composed of granite and other igneous rocks.

* The Rocky Mountains are a mix of igneous, metamorphic, and sedimentary rocks, with limestone being a minor component.

So, while some mountains are made of limestone, it's not the only type of rock that forms mountains.