The narrator refers to "the prodigy in [her]" as a child when she imagined all the possibilities for her fame-filled, glorious future, possibilities fed by her mother's urging and insistence. This prodigy becomes a metaphor for the narrator's inner feelings. However, the more her mother tested her, the more she failed: naming capitals, doing multiplication tables, saying everything she remembered from a page in the Bible, and so on. She began to feel that "something inside [her] began to die" because of all the disappointment she so clearly saw on her mother's face.
Soon, she looks in the mirror and feels as though she can see the prodigy's reflection, and "The girl staring back at [her] was angry, powerful. . . . [She] had new thoughts, willful thoughts—or rather, thoughts filled with lots of won'ts." It is not a prodigy inside her; rather, the prodigy is a metaphor for her feelings of rebellion. She learned to fight back against her mother, to be disobedient, and do what she wanted, instead of what her mother told her to do. As the years pass, this rebel "assert[s] [her] will, [her] right to fall short of expectations." Even though she might have been a pretty good piano player, it was more important to not try, "not to be anybody different." It turns out, then, that she really is a prodigy, a prodigy of rebellion! She practices and practices, and eventually wins the trophy of the piano, a testament to her powerful will.