Explain how the cobbles and pebbles that were transported by these streams become smooth and rounded in shape?

As the cobbles and pebbles are carried downstream by the stream, they constantly collide with each other and with the streambed, causing abrasion. This abrasion gradually wears away the rough edges and corners of the rocks, resulting in their smooth and rounded appearance. The longer the rocks are transported by the stream, the more worn down they become.