Here's why:

* Normal faults are fractures in the Earth's crust where the hanging wall (the block above the fault) moves down relative to the footwall (the block below the fault).

* This downward movement creates a tilted block, often with a steep scarp face, which forms the mountain.

* The process of normal faulting is often associated with extensional forces, where the crust is being pulled apart. This can happen in areas where tectonic plates are moving away from each other (divergent plate boundaries) or where the crust is being stretched.

Example: The Sierra Nevada Mountains in California are a classic example of fault-block mountains formed by normal faulting.