To build the windmill and keep the farm running at the same time, the animals have to work like slaves, enduring long hours. The animals make the sacrifice happily, since it's for their own benefit rather than for a human master. Boxer works the longest and hardest. Despite all the effort, the time spent working on the windmill makes the harvest slightly worse than it was the previous year.
The pigs' deception has worked. The animals still believe they are free even though they have no freedom at all. The pigs use Animalism to get the animals to sacrifice for the pigs' benefit.
The farm suffers shortages of items it can't produce itself, like nails and iron. Napoleon announces Animal Farm will start trading hay, wheat, and possibly even eggs with its neighbors, not for commercial uses, but for the benefit of the windmill. The animals are uneasy because they had agreed never to use money. Napoleon hires a man named Mr. Whymper to represent the farm, while Squealer convinces everyone that no rule ever banned the use of money.
To be fair, it's not clear how Animal Farm would get necessary supplies without resorting to trade. But the pigs don't explain the issue and then slightly modify Animalism. Instead, they lie, and use the threat of force to get the other animals to accept the lie.
Though the humans outside Animal Farm still hate and fear it, they also develop a grudging respect for the farm's efficiency.
Humans start accepting the farm as the pigs act more like humans.
Squealer begins to refer to Napoleon as "The Leader" and the pigs move into the farmhouse and begin sleeping in beds. Though Boxer dismisses the change by saying "Napoleon is always right," Clover is certain that the Commandments ban beds. She gets Muriel to read her the Fourth Commandment, but they discover that they seem to have misremembered it. It says: "No animal shall sleep in a bed with sheets."
Note the pigs' manipulation of language to increase their power and luxury. They take more privileges for themselves and justify those privileges by secretly rewriting the Commandments of Animalism.
By October the animals grow tired. Since the farm had to sell some food for money, the winter promises to be bad. The windmill is now half-finished, which pleases everyone (except Benjamin).
By manipulating Animalism rather than overthrowing it, the pigs exploit the animals without angering them.
One night in November, a terrible storm knocks down the windmill. The animals are horrified. Napoleon sniffs around the rubble. He looks worried and his tail wiggles as if he's thinking fast. Suddenly Napoleon shouts "SNOWBALL!" He announces that Snowball destroyed the windmill. The animals are shocked and furious that Snowball could do such a thing. Napoleon vows they will start rebuilding the windmill that very day.
The destruction of the windmill shocks Napoleon: he fears the animals might question his authority now. To save himself, he cleverly shifts the blame to Snowball. Stalin demonized the exiled Trotsky in a similar way.