Henry wants to make sure that his older sister Harriet never gets more of anything than he does so he carefully keeps score until Harriet gets a cavity and he gets none and Henry discovers that sometimes zero is better than one.
Henry is obsessed with getting as much as his sister till it comes to the dentist and he realizes he doesn't want as much as her (cavities) and takes a step towards kindness when he let's her get one more kiss, at night, too. Good evolution and math in fiction. My son read this to me when he was in first grade.