A difficult and contentious phenomenon to define, life is usually considered to be a characteristic of organisms that exhibit certain biological processes (such as chemical reactions or other events that results in a transformation), and that are capable of growth through metabolism and are capable of reproduction. The ability to ingest food and excrete waste are also sometimes considered requirements of life (e.g. bacteria are usually considered to be alive, whereas simpler viruses, which do not feed or excrete, are not). The two distinguishing features of living systems are sometimes considered to be complexity and organization (negative entropy). Some organisms can communicate, and many can adapt to their environment through internally generated changes, although these are not universally considered prerequisites for life.