At the end of chapter 6, the windmill is destroyed by a particularly fierce storm. The storm blows down the flagstaff and tears an elm tree from the ground as if it "had been plucked up like a radish." Napoleon, sensing an opportunity, blames Snowball, insisting that "Snowball has done this thing!" This is a case of dramatic irony: we, the readers, are aware that the storm is responsible for the collapse of the windmill, not Snowball. Snowball, however, is a convenient scapegoat. By pretending that Snowball is responsible for everything that goes wrong on the farm, Napoleon can distance himself from any responsibility, and he can also present to the animals an enemy against whom they can all unite. Presenting Snowball as the enemy and the cause of all that goes wrong also allows Napoleon to set himself up as the savior, the one who can fix the problems supposedly caused by Snowball.
In chapter 7, Orwell writes that:
Out of spite, the human beings pretended not to believe that it was Snowball who had destroyed the windmill; they said that it had fallen down because the walls were too thin.
Orwell here is adopting the point of view of the animals, as he does often throughout the story. From their point of view, the humans seem to be pretending not to believe that Snowball is responsible for the collapse of the windmill. The humans only seem to be pretending, however, because the animals wrongly believe Napoleon's lie that Snowball is responsible. The humans know better than to believe Napoleon's lies. For all of their faults, the humans are more intelligent and less gullible than the animals. The humans know that the storm destroyed the windmill because the windmill's walls really had been built "too thin."