I was looking at a small well-rounded stone on the beach yesterday.
If I took that stone and rolled it once, I wouldn't be able to perceive any change to it. Yet that stone was part of a much larger piece of rock. It was broken off, then rolled until it became smooth. At times, that stone likely sat still for thousands, hundreds of thousands, or even millions of years without being moved; but it was moved enough to be worn smooth and round yesterday.
Remind me to get your kid a rock tumbler for christmas. You'll discover that rounded pebbles are mere weeks away with the right abrasive... which leads me to conclude that rounded pebbles are the product of decades, not eons.
Beach glass is quick, man.