This is covered by:
AQA 8035, Cambridge IGCSE, CEA, Edexcel A, Edexcel B, Eduqas A, OCR A, OCR B, WJEC
Attrition
Attrition is an erosional process in which material is broken down by smashing into other material. On a beach this could be pebbles being flung against other pebbles, or pebbles being hurled against the base of a cliff or a sea wall.
When the materials collide they break, or pieces chip off them, and they become smaller. The process also smooths off their edges, making them rounder and smoother, which is why pebbles are round and smooth.