Marsupial mammals are best known for the belly pouch of many mother marsupials, such as kangaroos. The pouch is an adaptation to keep marsupial offspring safe, since they are very immature at birth. After no more than a handful of weeks of gestation in the uterus, newborn marsupials' hearts and lungs not fully developed. They may be as small as just a few millimeters (1 inch) long and skinny like a pinky finger. In species with a belly pouch, these puny young have strong front legs for crawling up to and into the pouch. Once there, they do what all mammals do--nurse on the mother's milk. The nourishment supports further development of their bodies, and teeth emerge. Their jaws house more cutting teeth (incisors) on the top than on the bottom, an arrangement that sets apart marsupials from other mammals.