Here's why:

* Conservation of mass states that the total mass of a closed system remains constant over time, regardless of changes within the system.

* In this scenario, the large rock is the closed system.

* Even though the rock undergoes a physical change (weathering), the total mass of the rock (the original rock plus all the tiny pieces) remains the same.

* The mass is simply distributed differently.

Let me know if you'd like more examples!