Dr. Sullivan explains that dermatology is the science of skin and its diseases. Skin, covering approximately 18 square feet, is a human's largest organ and serves as the body's protection. Using both live animals from the Baltimore Zoo and representations of animals, Dr. Sullivan explains their protective mechanisms: the lizard's skin changes color for camouflage; the duck's preen gland maintains its feathers with a precursor of vitamin D; the rat's sebaceous glands repel lice; the lion's mane protects it from other animals' bites; and a turtle's shell, a porcupine's quills, and a pachyderm's thick skin all protect the animal within. Dr. Sullivan draws a diagram of the layers of human skin, stressing the epidermis and protective barrier that prevents water from penetrating skin. He notes that a frog has no transitional layer, so its skin can take up water. The human sweat glands continuously secrete a wet film on the skin. The sebaceous glands secrete sebum, which contains two emulsifiers that allow the skin to sweat.