How does Lennie get in an argument with Curley? What happens as a result of this argument?

When Curley gets into an argument with Slim over his wife, Lennie makes the mistake of having a smile on his face. Note, the smile has nothing to do with the situation.... Lennie is merely dreaming about the rabbits. Unfortunately, Curley thinks that Lennie is laughing at him and challenges Lennie, who egged on by George accepts the challenge and crushes Lennie's hand.