What event sparked the animals' rebellion in Animal Farm?
To answer this question, take a look at chapter 2. According to the narrator, Mr. Jones went to the Red Lion pub at Willingdon on a Saturday. He got drunk and did not come home until the next day, after which he promptly fell asleep. In his absence, the farm workers have milked the cows and then gone out rabbiting. What is important here is that neither Mr. Jones nor his farm workers fed the animals. The animals have been without food for over twenty-four hours.
As a result of their hunger, one of the cows breaks down the door of the farm store shed. This allows the animals to help themselves to food from the bins.
It is at this moment that Mr. Jones and his men find the animals. Enraged by the fact that they are helping themselves, the humans try to control the situation by lashing at the animals with their whips. The animals, however, are not prepared to put up with this treatment. They fight back, causing the humans to flee the farm.
What is interesting about the rebellion is that it happens much sooner and much easier than expected. The animals are forced to act because of Mr. Jones's maltreatment and neglect. Had Mr. Jones not acted in this way, the rebellion might never have taken place at all.
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Old Major's speech put the idea of rebellion in the animals' minds, but they didn't have any idea when it would begin or whether "it would be within their own lifetime." Snowball and Napoleon took to organizing the others, so the animals were ready when the time came. Then, on Midsummer's Eve (in June), Mr. Jones got so drunk that he didn't come home until the next day. After arriving, he went right to sleep without feeding the animals. One of the cows broke down the door to the shed, so Jones' workers began "lashing out in all directions" with their whips. In one sudden, unified rush, the animals attacked the men, who were "butted and kicked from all sides." Mrs. Jones grabbed a few items and ran from the house, joining her husband and the workers in the road, where the animals left them, slamming the gate behind them.