Final answer:
Rock particles in a stream become rounded due to erosion, where the collision with other particles and abrasion wears down their edges, especially as they travel longer distances.
Step-by-step explanation:
Rock particles transported by a stream often become more rounded due to a process called erosion. As rocks and sediments are eroded from their place of origin and carried by the flow of the stream, they collide with other particles, the streambed, and the banks.
The distancing from the source is also a factor in the degree of rounding. Fragments that travel longer distances generally show greater signs of wear and tend to be better rounded than those that haven't travelled far from their point of origin.