The animals use gravity to break up the stone for the windmill by throwing boulders over the edge of the quarry.
When the animals decide to build the windmill, they know there is a “good quarry of limestone on the farm” and plenty of sand and cement that “had been found in one of the outhouses” (ch 6). The animals are not able to use human tools such as “picks and crowbars” because they cannot yet stand on their hind legs. So they decide to “utilise the force of gravity.”
There were huge boulders lying everywhere, so the animals figured out a way to break them.
The animals lashed ropes round [the boulders and] dragged them with desperate slowness up the slope to the top of the quarry, where they were toppled over the edge, to shatter to pieces below. (ch 6)
All of the animals helped. Once the boulders were broken into smaller pieces of stone, it was easy to pick them up and transport them in the wagon to the building site.
The building of the windmill is an example of accomplishment and progress for the animals. They decide that they are going to get electricity, they figure out how the build the windmill, and they build it. Yet the windmill also serves as a distraction. It keeps the animals busy so they don’t see what the pigs are really doing.