The Key to Success
Okay, we're being a little optimistic with our headline. The Key to Nothing would be a more appropriate title, but for most of the book, the key's a symbol of hope for Oskar. Unable to climb out of grief for his father, he finds a sense of purpose in tracking down the key. It frees him from his daily routine, which is depressing and boring to him. He treks all over the city, meeting new people and getting a broader perspective. He hopes the key will give him some special insight into his father. While he's searching, his father's with him in spirit. Well, Oskar wouldn't say that. He'd say that the search keeps his father in mind. He says that the more he searches, the closer he feels to Dad.
When Oskar finds the key in a vase in his Dad's room, he wears it "like a pendant" (3.72) over his heart, and is determined to find what it goes to. When it begins to irritate him, he puts a Band-Aid over his heart. We couldn't think of a better image here than a Band-Aid over the heart. But the quest is just a Band-Aid. He pulls it off in the end, and has to confront reality: that his Dad is dead and nothing will ever be the same. All you can do is love the people who are left.
The puzzle of the key is reminiscent of the Reconnaissance Missions that Oskar's Dad sent him on, hoping to open up the larger world to Oskar. The new journey does that. Mission accomplished, Dad.