Pebbles come in all shapes and sizes. Pebbles with extrusions are more likely to hit another object, or be hit against by an object, in a way that causes wearing. Wearing can cause a jagged edge to become rounded, or a rounded pebble to become jagged again, but rounded edges are harder to turn jagged than vice versa. Over time, the distribution of shapes will be dominated by roundness.Or the rounding of pebbles in a streambed or ancient desert.
You really shouldn't need me to explain how a sand-sifter is an example of selection.