Here's why:

* Erosion is the process of wearing away and transporting rock and soil. In this case, the force of the water in the stream is what's causing the rocks to be worn down and smoothed.

* Weathering is the process of breaking down rocks, but it doesn't involve movement. While weathering might play a role in creating the initial cracks and weaknesses in the rocks, the actual smoothing process is due to the abrasive action of the water and the rocks themselves.

Let me know if you'd like to know more about erosion!