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As Clover looked down the hillside her eyes filled with tears. If she could have spoken her thoughts; it would have been to say that this was not what they had aimed at when they had set themselves years ago to work for the overthrow of the human race. These scenes of terror and slaughter were not what they had looked forward to on that night when old Major first stirred them to rebellion. (7.30)
Notice how the key phrase in this quote is "If she could have spoken her thoughts." The point is that Clover vaguely senses that something is wrong, but she cannot articulate it and so she remains loyal to the pigs. The pigs are extremely reliant on the inability of the animals to articulate their thoughts.
Frightened though they were, some of the animals might possibly have protested, but at this moment the sheep set up their usual bleating of "Four legs good, two legs bad," which went on for several minutes and put an end to the discussion. (7.36)
The literal noise of the sheep silences the animals, just as metaphorically the bland repetition of a simple phrase silences any counterargument.
One day in early summer Squealer ordered the sheep to follow him, and led them out to a piece of waste ground at the other end of the farm, which had become overgrown with birch saplings. The sheep spent the whole day there browsing at the leaves under Squealer's supervision. In the evening he returned to the farmhouse himself, but, as it was warm weather, told the sheep to stay where they were. It ended by their remaining there for a whole week, during which time the other animals saw nothing of them. Squealer was with them for the greater part of every day. He was, he said, teaching them to sing a new song, for which privacy was needed. (10.8)
Squealer separates the sheep from the other animals to use them as tools, just as Napoleon separated the pups when they were born.