Living things are complex.Their beings are comprised of one or more cells, the microscopic building blocks of any living thing. Cells that join forces to complete a shared task form tissue. Tissues form organs which work together to form systems of organs. Organisms are characterized by their complex systems of organs.
Living organisms process nutrients from the atmosphere, like air, food or sunlight, and expel or use the chemical energy. This is called metabolism. Living things have a metabolism and non-living things do not.
Living things are able to respond and adapt to external factors and stimuli. Non-living things have no means by which to respond and do not adapt. Responsiveness is an active action, not passive. A ball rolling down a slope is passive. A person pulling their hand back after touching something hot is an active action. One characteristic that all living things share, no matter how simple or complex the organism, is the ability to respond.
Living things are able to grow while non-living things do not. Growth occurs when a living organization processes material that is different then they are and transforms it into material that is like them. A dog eating dog food, which is unlike them, turns it into material that is like them which aids in growth. Energy that was produced as part of the metabolism process is expended in the process of growth.
Reproduction occurs when a living thing produces a copy of itself while still living. For less complicated organisms, reproduction can be a continuation of the growing process. There are two types of reproduction, asexual and sexual. Asexual reproduction occurs when a single organism produces an offspring that has only the one parent and whose cells are exact replicas of the parent cells. Sexual reproduction occurs when two organisms contribute to the creation and traits of their offspring. Sexual reproduction is more complicated than asexual reproduction and usually involves some level of care for the offspring after it is produced. Non-living things do not reproduce.