The exposition is the section of the plot that introduces the story to the reader or audience. It presents us with the main character or characters and also points towards the main conflict that will act as the catalyst of the action of the story. It comes before the rising climax, where that conflict is brought into focus.
The exposition of this wonderful book therefore comes as we are introduced to our somewhat bizarre and quirky first-person narrator, Oskar, aged 12, who is certainly hyper and fascinated with strange and whacky inventions:
What about a teakettle? What if the spout opened and closed when the steam came out, so it would become a mouth, and it could whistle pretty melodies, or do Shakespeare, or just crack up with me?
As the story continues we understand more (though not all) about this young boy and the key that he finds and what has happened to his father, and how he feels he is on a quest to discover what the key was meant to open. This is the challenge that drives the rest of the story and brings this lovable character into the lives of so many others.