Fault block mountains form when large blocks of crust are lifted up along faults. Here's a breakdown:

* Faults: These are fractures in the Earth's crust where rocks have moved past each other.

* Lifting: The movement along the fault causes one block of crust to be pushed upward relative to the other.

* Blocks: These are large sections of the Earth's crust that have been broken apart by the fault.

In simpler terms: Imagine two large pieces of a puzzle. One piece is lifted up above the other along a crack (the fault). The lifted piece becomes a mountain, and the lowered piece forms a valley.

Here's a common example: The Sierra Nevada mountains in California were formed by fault block mountains.