Why did someone write "fat girls" on the restaurant check?

What are people thinking? Yeah, that’s rhetorical, but I’m also serious. How do people think when they do things like that. Here’s a story about it.

Did you ever use a slur about a person? Can you reconstruct your thinking that made you say that? Is this a matter of lack of awareness, or is it simple meanness, or cultural acceptance, lack of thought or what?