You are absolutely right! Weathering is the primary cause of rocks in a stream becoming smooth. Here's how it works:

* Abrasion: As rocks are tumbled along the stream bed, they collide with each other and the streambed itself. This constant friction wears away rough edges and sharp points, gradually smoothing the rock surfaces. Think of it like sandpapering.

* Chemical Weathering: Water itself can chemically react with some types of rock, dissolving minerals and softening them. This process also contributes to rounding the rock's shape.

* Biological Weathering: Even organisms like lichens and algae can contribute by secreting acids that erode the rock.

So, in a nutshell, it's the combination of physical and chemical processes over time that turns rough, jagged rocks into the smooth, rounded pebbles we see in streams.