In a stream or river, water flowing over stones causes the stones to be rounded and smooth due to

The process that causes stones in a stream or river to become rounded and smooth is called erosion. Here's how it happens, step by step:

1. Water Flow: The water in a stream or river is constantly moving. This movement is what initiates the process of erosion.

2. Collision: As the water flows, it causes the stones to collide with each other. These collisions can chip off small pieces of the stones.

3. Friction: The constant movement of water also creates friction between the water and the stones. This friction can wear down the surface of the stones, smoothing out any rough edges.

4. Sediment Transport: The small pieces of stone that are chipped off become part of the water's sediment load, which is the material carried by the water. This sediment can further contribute to the erosion process as it continues to collide with the stones.

5. Continual Erosion: Over time, the continual process of collision, friction, and sediment transport can significantly alter the shape of the stones, rounding them and smoothing their surfaces.

This process is a fundamental part of the natural shaping of our landscapes, and it's why you'll often find smooth, rounded stones in or near bodies of flowing water.