A recovering curmudgeon narrates life lessons in the latest entry in the punny Food Group series. Grape wasn't always sour, as they explain in this origin story. Grape's arc starts with an idyllic childhood within "a close-knit bunch" in a community of "about three thousand." The sweet-to-sour switch begins when Grape plans an elaborate birthday party to which no one shows up. Going from "sweet" to "bitter," "snappy," and, finally, "sour," Grape "scowled so much that my face got all squishy." Minor grudges become major. An aha moment occurs when a run of bad luck makes Grape three hours late for a meetup with best friend Lenny, who's just as acidic as Grape. After the irate lemon storms off, Grape recognizes their own behavior in Lenny. Alone, Grape begins to enjoy the charms of a lovely evening. Once home, the fruit browses through a box of memorabilia, discovering that the old birthday party invitation provided the wrong date! "I realized nobody's perfect. Not even me." Remaining pages reverse the downturn as Grape observes that minor setbacks are easily weathered when the emphasis is on talking, listening, and working things out. Oswald's signature illustrations depict Grape and company with big eyes and tiny limbs. The best sight gag occurs early: Grape's grandparents are depicted as elegant raisins. The lessons are as valuable as in previous outings, and kids won't mind the slight preachiness. (This book was reviewed digitally.) Sweet, good-hearted fun. (Picture book. 4-8) Copyright (c) Kirkus Reviews, used with permission.