Living and nonliving things bear commonalities in that they are both comprised of matter, obey physical laws and tend toward states of minimum energy. Living things are different in that they can repair damage, grow and reproduce.
Living things exhibit basic characteristics such as feeding — the intake of energy from the environment — to enable other functions. A single living thing is called an organism. Growth increases the size and complexity of the organism due to the creation of bigger and more robust physical structures.
Breathing involves the exchange of gases with the organism's environment. Many organisms intake oxygen and expel carbon dioxide as their main respiratory process, although plants intake carbon dioxide and expel oxygen during photosynthesis. All organisms also excrete, removing excess and waste products out of the body.
Organisms must also be sensitive to environmental triggers such as light, heat, touch and sound. Different organisms are sensitive to different triggers, determined by the conditions needed for survival.
Organisms are also capable of reproduction. Simple organisms self-replicate, producing almost exact copies of themselves, while more complex organisms reproduce sexually, producing offspring containing combinations of the genetic traits of both parents. Some organisms are also capable of self-powered movement, such as walking, swimming and flying.
Nonliving things may be capable of some of the feats of living things, but unless they can autonomously do all of the above, they cannot be qualified as living. Viruses are often classified as nonliving because they cannot reproduce on their own.