There is an excellent analysis of this short story here on eNotes that can help you with this assignment. When you read the overview of the story, you will see that the family and the guest are more than just characters. They represent the author's theme. The family is a unit, but it consists of individuals. The unit's function is community, nurture and love, in spite of the fact that each individual in the family is unique. In contrast, the guest is alone. He is unique but he has only his solitariness and does not exist as part of a family. Therefore, he has no community. The words of the family and the guest illustrate this point. The family is content with what they have until the guest shows up and starts talking about his "ambition" - but it is just that - talk. When the slide comes and destroys the people, the greater community remembers the family AND its individual members by the tokens of them that have been left behind. There is no evidence that the guest ever existed.

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