How do sediments eroded by the water in a stream become smoother and rounder in shape?

Sediments eroded by water in a stream become smoother and rounder through a process called abrasion. As the sediments are transported by the flowing water, they collide with each other and with the streambed, causing their edges to wear away. This constant friction gradually reduces sharp edges and corners, resulting in more rounded and polished shapes. Over time, the continuous movement of water further enhances this smoothing effect, leading to the characteristic rounded appearance of river rocks.