"A dad and two kids walk their English setter and explore what distinguishes mammals from all the other animals they see. Beginning by establishing that animals "can eat, breathe, move, and grow" but that not all animals are mammals, the book introduces several members of the animal kingdom in reverse order of their proximity to mammals on the taxonomic tree. An earthworm, for instance, is an animal, but it "is soft inside and out," whereas mammals have "some body parts that are hard." Similarly, ladybugs have hard body parts, but they're only on the outside, unlike mammals', which are "mostly on the inside." Thus largely eschewing scientific vocabulary in favor of clear explanations (the terms "vertebrates" and "invertebrates" are introduced in a diagram in the backmatter), Rockwell's text focuses on the concepts. The fine-lined ink-and-watercolor illustrations are as clear and straightforward as the text, with the carefully labeled renderings of the animals examined particularly meticulous. Occasional, supplementary text in a smaller type provides further information, such as the facts that whales breathe through blowholes and "snakes usually have just one long lung." The family is an interracial one, with a white dad and brown-skinned mom who is seen nursing a baby in the final spread, underscoring humanity's kinship with our fellow mammals.

A clear, respectful introduction. (further facts, references)" -- Kirkus Reviews "In this gently illustrated companion to A Bird Is a Bird , a father takes his son and daughter on a walk with their dog through the farm and woodland near their home. As they explore, Rockwell introduces the characteristics of mammals (such as having a skeleton and spine, being warm-blooded, and breathing air) and asks readers whether certain animals meet the criteria: "A ladybug is an animal. A ladybug has body parts that are hard. But is a ladybug a mammal? No!" Animals that do fit the bill include humpback whales, harbor seals, white-headed capuchin monkeys, and African elephants. While readers may need clarification that not all of the featured animals share a habitat--and don't all live in places like the area the characters are exploring--they'll learn plenty about the ways that animals are categorized and classified." --Publishers Weekly.