Here's how:

* Differential Erosion: Waterfalls form when a river or stream flows over a layer of rock that is more resistant to erosion than the rock beneath it. The softer rock erodes faster, creating a drop or cliff.

* Headward Erosion: The force of the water over the waterfall continues to erode the resistant rock, causing the waterfall to gradually move upstream.

* Undercutting: The water's force also erodes the rock at the base of the waterfall, creating a "plunge pool." This undercutting can eventually lead to the collapse of the waterfall's edge.

So, in short, waterfalls are the result of water wearing away (eroding) the rock over time.