Here's why:

* Volcanoes are mountains: Volcanoes are formed by the accumulation of lava, ash, and rock that erupts from the Earth's crust. This buildup over time creates a cone-shaped structure, which is essentially a mountain.

* Not all mountains are volcanoes: While all volcanoes are mountains, not all mountains are volcanoes. Many mountains are formed by other geological processes like tectonic plate collisions or erosion.

* Examples: Mount Fuji (Japan), Mount Vesuvius (Italy), and Mount Kilimanjaro (Tanzania) are all examples of mountains that are also volcanoes.

So, while a mountain can be a volcano, not all mountains are volcanoes.