Here's why:

* Convergent Plate Boundaries: These are areas where two tectonic plates collide. The collision forces can cause one plate to slide under the other (subduction), or they can push against each other, causing the land to buckle and fold upwards.

* Subduction Zones: When one plate dives under the other (subducts), it can cause the overriding plate to buckle and fold, forming mountain ranges. The subducted plate also melts, creating magma that rises to the surface, often leading to volcanic activity. The Andes Mountains in South America are a prime example of mountains formed by subduction.

* Continental Collision: When two continental plates collide, neither is dense enough to subduct. The immense force of the collision causes the land to crumple and fold, creating massive mountain ranges. The Himalayas, the world's tallest mountain range, formed from the collision of the Indian and Eurasian plates.

In summary: Mountain formation is a result of tectonic plate movement, specifically the collision of convergent plates. The force of the collision, either through subduction or continental collision, causes the land to fold, buckle, and uplift, creating the majestic mountain ranges we see around the world.