You shouldn't say things without thinking.
Marcy is at a party and is goofing off with her friends - one of them is overweight.
Marcy wants to make people laugh.
Marcy thinks of many funny things she could say and decides to tell a funny story about her cat.
Marcy says the first thing that comes to her mind, which happens to be a joke about fat people.
All of Marcy's friends, including the overweight one, have a good time and laugh at the story.
Marcy's overweight friend is deeply hurt by the joke.