Here's how it works:

* Wave action: The constant movement of waves carries sand, pebbles, and other debris.

* Friction: These materials rub against the shell, gradually wearing down its rough edges and sharp points.

* Polishing: Over time, the continuous friction polishes the shell, creating a smooth, rounded surface.

So, while the shell isn't directly eroded by the waves, it's the abrasive action of the sand and other particles carried by the waves that gives it its smooth appearance.