The exposition of a story provides critical details for a reader in the opening of a story. Therefore, a exposition (or introduction) introduces characters, setting, and plot (or action).
In Shirley Jackson's short story "Charles", the exposition contains Laurie's mother's description of her son prior to kindergarten through Laurie's introduction of Charles. The true ending of the exposition happens after the following line is spoken by Laurie's father:
"See here, young man."
After this point in the story, the information about Charles compounds. Charles is constantly getting into trouble. Therefore, it is the initial introduction to Charles, and the spanking that he receives from the teacher, which is where the story begins to move into the rising action given the initial conflict (Charles being a bad boy) has already been introduced.