Why Do Cobblestones Have a Smooth Surface?

On some river banks or sea beaches, we can find many smooth, flat and round stones as big as egg or walnuts. People call them cobblestones.

Why do all cobblestones have a smooth surface?

On some mountains by the banks of rivers or at the seaside, there are a lot of rocks. After being exposed to the sun or rain, big rocks become small pieces of stones. Later these stones are flushed into rivers and seas by floods. After many, many years of scouring, these sharp stones are polished by the water and they become very smooth cobblestones.