in chapter V
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By this point Napoleon's domination is complete. Napoleon tells the animals they are going to build the windmill and takes credit for the idea. Napoleon’s guard dogs growl and frighten the animals followed by Squealer's insistence that Napoleon only speaks the truth.
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Napoleon was a scheming pig, and so the construction of the windmill was obviously advantaged towards himself. By making the animals construct the windmill, he could establish rationing for the animals, seemingly because the manpower for the harvesting would be decreased by half, but in reality to allow him to have more food to eat. He could also occupy the animals with something to do, as seen when the windmill topples over on various occasions, he makes them continue to make it until it would not fall. This would be useful in providing a vague memory to the animals and it would be easier to manipulate them.