They swallow it like grain

In 1984 and Animal Farm, the societies couldn't stay the way they were without propoganda. To put it gentley, the people were told things that weren't quite true, but who wants to put it gentley? The leaders would blatently lie to their people to keep them in line.
The pigs did it constantly when they changed their commandments for their won good. Squealer would always come out to shovel the lies out to the other animals. When they changed the rule where "No Animal Shall Sleep In a Bed With Sheets" and some of the animals where trying to remember if it had always been that way, Squealer would set them straight by saying that this had always been the way and their memories were wrong. How did they just accept that their memories were wrong and what someone else was telling them was true? As Orwell explains many times, some of the animals are not very bright so they would believe anything they were told since the pigs were seen as the smartest animals on the farm. With the pigs being the smartest, all of the other animals just accepted everything they said as absolute truth, even though it was absolute nonsense. With the animals, it is kind of understandable that they trust the pigs so much because they are told that the pigs are superior to all other animals on the farm. They even convince the animals that they need all of the apples and milk to themselves for their mash because they need the nourishment for all of the extra brainpower that they have to do. Of course it is obsurd to think that the pigs need more food than the other animals that slave in the hot sun all day while they sit in cool buildings thinking, but the animals don't know any better and trust the pigs because they are smarter.
In the end, all of the lies add up to an obedient group of citizens that do all of the work while the liers reap all of the benifits. Then they become exactly what they were trying to avoid: unmerciful tyrants.