Since the tenets of Animalism were too hard for the animals to memorize, Snowball shortened them to "Four legs good, two legs bad." When the chickens inquired further, they were told that for Animal Farm's purposes, their wings could count as legs, thus making them fall into the "good" category. The sheep soon picked up the chant, "Four legs good, two legs bad," and a motto for the farm was born.
This attempt at humor by Orwell also demonstrates that the animals were incapable of abstract thought about their own governance. The animals did not only have to have a mantra, but they had to have it chanted over and over. When Napoleon exiled Snowball, he used the chant in order to drown out any anti-animalism sentiment.