Encyclopedia Brown is asked to investigate the case of the missing ring. The ring belongs to Sally Kimball, and she lost it while she was playing in the park. Encyclopedia Brown interviews Sally and her friends, and he also searches the park. He finds no clues to the ring's whereabouts.
The next day, Encyclopedia Brown goes to Sally's house. He sees that Sally is eating a pickle from a jar. He asks Sally where she got the pickle, and she tells him that her mother bought it at the store. Encyclopedia Brown looks at the pickle jar, and he sees that there is a ring in it. He takes the ring out of the jar and gives it to Sally.
Sally is very happy to get her ring back. She asks Encyclopedia Brown how he knew that the ring was in the pickle jar. Encyclopedia Brown tells her that he remembered that Sally had been eating a pickle from the jar when he interviewed her the day before. He also remembered that Sally had said that her mother had bought the pickle at the store. Encyclopedia Brown figured that if Sally's mother had bought the pickle at the store, then the ring must have been in the pickle jar when she bought it.