Human beings are vertebrate, mammal, viviparous, omnivorous land animals that are distinguished from other animals by their brains, skulls, ability to walk upright on two legs (bipedalism), opposable thumbs, and skin. Humans go through various stages of life from birth to old age, being dependent as babies, learning as children, maturing as adolescents, reproducing as adults, and retiring as elders. Key human biological systems include nutrition through eating, drinking, and breathing and interaction through the senses of sight, smell, hearing, taste, and touch.