In the book Rules of the Game, what happens early in the story when Waverly asks for a bag of salted plums?

When Waverly's out shopping with her mom one day, she asks for a bag of salted plums. A perfectly reasonable request, one might think. But Waverly's mother flatly refuses. At this, Waverly throws a huge tantrum, a hissy fit of epic proportions. But her mother still won't back down. Waverly's mother wants to teach her a lesson: the importance of exercising self-control, and of biting your tongue.
And Waverly seems to have learned her lesson, because the next time she goes shopping with her mom she bites her tongue and says nothing. This time Waverly's mother buys her the salted plums; this is a reward for Waverly's good behavior. She's clearly learned how to control herself, bite her tongue, and in the process began to understand the power of invisible strength.