Living things are organisms that display the key characteristics of life. These characteristics include the ability to grow, reproduce, take in and use energy, excrete waste, respond to the environment, and possess an organized structure more complex than that of non-living things.
Sometimes movement is also listed as a key characteristic of life, but many living things, such as trees, mushrooms, barnacles and coral, are incapable of movement during some or all of their life stages. Viruses, on the other hand, are a special case of an organism that is not a living thing. Although viruses are unable to reproduce or take in energy on their own, they are classified as organisms because they still contain genetic data and have evolutionary lineages. Furthermore, viruses are capable of reproducing within host cells and exist independently of their hosts, even if they are inanimate outside of them. Viruses are only considered living by scientists who consider the only key characteristic of life as the ability to replicate.