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What major problem initially keeps the animals from building the windmill?
There is no quarry on the farm and they can't find anywhere to obtain stone.
The rocks are too large and none of the animals could use tools to break them up.
The stone is too heavy to be moved up the hill to the windmill site.
The pigs are initially unwilling to help pull the loads of rock, and without their strength the rocks can't be moved.
Nothing - there is no major problem.