Symbolism, Imagery, Allegory
Victor goes into his parents' bedroom because he's feeling bored and like he wants to snoop around (come on—we've all been there), but what he finds in their closet changes his life drastically. See, Victor finds his birth certificate, and when he takes a closer look at it, he finds a shocking secret about his family:
Inside was a small piece of paper that said "Birth Certificate Of" and the name written in longhand was "Baby Boy." The date on it was my birthday. My mother's name before she got married was written at the bottom of the paper on the left side beside MOTHER. On the right side next to FATHER was a word I wasn't expecting. (7.76-77)
The birth certificate marks a turning point in Victor's life. Before finding it, he's a kid who's innocent to any tensions and dramas that might be lurking beneath the surface of his idyllic family. He takes everything at face value. But when he learns that his father isn't biologically related to him, he has to decide what family really means to him. In the end, though, Victor thinks long and hard and decides that the birth certificate isn't all that important; what really matters is that his father loves him:
My father on the birth certificate might have been Unknown but the tall man throwing ball with me in his white shirt with his necktie stuffed between the buttons was my father as far as I was concerned. (19.73)
Although the birth certificate still does change things for Victor, one thing that it doesn't affect is his relationship with his father. They still love and care about each other—and that's not going to change no matter what a flimsy piece of paper says. Coming to this understanding is just another way that Victor comes to trust himself and be his own person.