Kangaroos are mammals with special physiological characteristics. One of their characteristics is that female kangaroos have a skin bag that can hold their young, which is called "kangaroo bag". The kangaroo bag is located between the abdomen and hind limbs of the female kangaroo, and the opening is upward. The kangaroo bag is shaped like an upward curved arc, and looks like the number 0 in appearance. This special kangaroo bag is very important to the female kangaroo. It provides an environment for protecting and incubating their offspring. Cubs are very small and fragile after birth. They will climb into the mother bag and get nutrition and protection in the kangaroo bag until they are mature and can leave the kangaroo bag.