As I walked home, I thought about the message. In the course of a lifetime, what does it matter? I said it over and over. I wondered about the mysterious messenger, and I thought about all the things in the course of a lifetime that would not matter. I did not think cheerleading tryouts would matter, but I was not so sure about yelling at your mother. I was certain, however, that if your mother left, it would be something that mattered in the whole long course of your lifetime.
This quote is also from Chapter 17. Sal walks home from Phoebe's, having witnessed Phoebe's angry refusal of Mrs. Winterbottom's brownies, Prudence's rejection of her mother's offers of support, and the appearance of another mysterious message. Sal reflects on her understanding that people, like Prudence with her cheerleading tryouts and Phoebe with her lunatic, tend to blow many of their insignificant concerns and preoccupations out of proportion. She guiltily remembers the times she acted rudely toward her mother, realizing that the way we treat people on a day to day basis actually does matter in the long run. Her conclusion, that a mother's departure can affect an entire life, suggests that Sal may blame herself for her mother's disappearance and foreshadows the disappearance of Mrs. Winterbottom.