What is the primary reason a smooth round stone became smooth?

A smooth round stone becomes smooth primarily due to prolonged exposure to water, particularly in a stream or river. As the stone tumbles and shifts with the current, the erosion process occurs. This natural process involves the water wearing away rough edges and surfaces, resulting in a smoother appearance over time. This is a gradual effect of mechanical weathering as the stone’s surface is continuously worn down.

Other factors such as being buried in soft soil or experiencing high temperatures do not contribute significantly to the formation of the smooth, rounded texture typical of stones found in streams. While polishing by human hands can create a smooth finish, the question specifically seeks to identify the natural process responsible for the typical smoothness of stones encountered in nature.