To answer this question, take a look at Chapter Five. According to the narrator, Napoleon takes a look at the detailed plans for the windmill and then urinates all over them before silently walking out.
This is a powerfully symbolic act. By urinating on the plans, Napoleon makes it clear that he feels nothing but contempt for the building of the windmill. He sees no value or point in its construction and will not support it.
Moreover, this is also an act of defiance and domination over Snowball. Napoleon is showing that he will not back down or be convinced by Snowball's plan nor will he let Snowball take control of the farm's direction.
Later in the chapter, Napoleon further expresses his contempt for the windmill by arguing in favor of increased food production and against the construction of the windmill. He also coins the slogan, "Vote for Napoleon and the full manger." Napoleon's contempt cannot be clearer and this sets the scene for his expulsion of Snowball later on.