As part of the rock cycle, rocks come to the surface, where they are exposed to the weather and other agents which break them into smaller pieces. This breaking up is called 'weathering'.
Weathering happens in many ways:
The sun heats rocks up and makes them crack. Water freezes inside cracks and makes them wider. Rain and wind wear little bits off. Living things grow in rocks and break them up. Rocks bump into each other which breaks them and wears them down.
Set up the rock tumbler with a variety of hard and soft rocks inside, to show what happens when rocks bump together.
While the rocks are tumbling (for about an hour), go on a weathering scavenger hunt.
Check inside the tumbler after about an hour, to find that the soft rocks have already worn away or broken up a bit.
Leave the tumbler for another week.
The soft (sedimentary) rocks might disappear completely, becoming mud, while the hard (igneous and metamorphic) rocks are only smoothed a little at the edges.
(Make sure that the tumbler turns all the while - as some of the rocks become mud, the tumbler might stop turning, and need a little more water adding.)
The rock tumbler mimics rocks colliding into each other, and are weathered into smaller, rounder rocks, and sand.
Weathering as part of the carbon cycle:
As weathered rock particles are exposed to the air, their carbon is converted to carbon dioxide, which becomes part of the atmospheric CO2.