Rodents are famous for their teeth. The large, paired cutting teeth (incisors) in the front of their mouth give them a buck-toothed appearance. The incisors are self-sharpening, thanks to the way they grind together during gnawing. While a strong enamel coating prevents wear on the front, the back of each incisor is soft and wears down. The outcome is a chisel-like shape that can cut into wood, food, or predators. The incisors grow continuously, creating tooth material for sharpening throughout the life of a rodent. A rodent skull is shaped to support the muscles used in gnawing, such as an enlarged region (the pterygoid) that anchors the lower jaw muscles. A skull modification allows the lower jaw to move not only up and down, but back and forth for better grinding action. Rodents are the largest group of mammals on Earth, and their ability to gnaw is part of the reason they have been so successful.

Mammals are named for their special milk-producing structures called mammary glands. A female mammal may have two (like humans) or as many as 18 mammary glands. Each gland is full of small spaces lined with milk-secreting cells that all drain into a nipple. Newborn mammals have an innate sucking reflex that stimulates the mother's mammary glands to release milk. Milk is a superfood for young mammals, containing a nourishing mix of fats, proteins, vitamins, minerals, and even immune-system chemicals. The first milk (colostrum) contains antibodies from the mother to protect against disease. As the offspring grows, milk changes to maintain the right balance of nutrients. Production of milk (lactation) makes most female mammals temporarily unable to get pregnant, resulting in natural gaps between offspring. The spacing of births benefits both the offspring and the mother in terms of survival. Mother mammals stop nursing (wean) their babies after several days or years, depending on the species and environmental conditions.