Marvin is the only person to like Rosie, the meanest cow on the dairy farm where Marvin's father works. When the farm and its stock are sold, Marvin's father finds a new job, and the family members reluctantly dissemble their lives, moving into a mobile home on a different farm. Marvin attributes his new, mean behavior to Rosie; he claims that the cow's new owner turned her into "the smallest cow in the world" and that she's right there with Marvin again because they both know it's no fun being little. After Marvin's parents and older sister accept the idea that a tiny Rosie is Marvin's imaginary playmate, the boy eases into their new life. Paterson's upbeat story shows a contemporary, blue-collar family making the best of changes not of their own making. Her appreciation for the complexity of a child's responses to changes he cannot control is matched by her gift in expressing the constructive, transforming power of imaginative play in a very few words. Paterson shows marvelous versatility in creating beginning fiction replete with requisite repetitions, yet never losing the strong sense of story and adroit turn of phrase so characteristic of her novels. (Ages 4-7)