Here's why:

* Erosion is a continuous process: Erosion is the gradual wearing away of rock by natural forces like wind, water, ice, and gravity. These forces constantly act on rocks, chipping away at them over time.

* No "limit" to erosion: There's no set point where a rock is "done" with erosion. As long as the forces of erosion are present, they will continue to wear away at the rock, no matter how small it gets.

* Fragmentation and new surfaces: As a rock is eroded, it can fragment into smaller pieces. Each of these fragments then becomes a new surface that is susceptible to further erosion.

Think of it like this: Imagine a mountain. Over millions of years, wind and rain will wear it down, eventually turning it into a valley. But those smaller pieces of rock that made up the mountain will still be subject to erosion, and they will continue to be eroded until they are eventually reduced to tiny particles of sand and silt.

So, while a rock can be dramatically changed by erosion, it never truly "stops" being eroded.