What does Oskar do with the key in this chapter?

In this chapter Oskar finds a key labeled black in his father's closet. Staying home from school, he goes to a locksmith, who gives him ideas about what the key might fit. Next morning, he goes to an art supply store to search for clues about the key. For eight months, he takes forays into the city to check out all the surname "Black" in the telephone book because that is written on the key. He feels closer to his dad during these outings, but far away from his mom.