Gather several green leaves (look for ones that have fallen to the ground). Place them in a resealable bag and seal.
Hold the bag between you hands and rub the bag between your hands 25 times. Observe the leaves.
Now add several small rocks the bag.
Rub the bag together 25 more times. Observe the leaves.
You'll find that the leaves without rocks did not change much at all. However, the leaves in the bag with the rocks were crushed.
Rocks found near the rib bones of some dinosaur skeleton suggest that those dinosaurs swallowed rocks to aid in the digestion of food in dinosaurs who lacked grinding teeth. Modern chickens swallow gravel to grind food inside their gizzard.