Biologists have certain criteria that must be met if something is to be considered living. All living things are made up of cells and have
, which carries the genetic code. Living things also
, either sexually or asexually. The chemical reactions of
help an organism use materials and energy to stay alive. Living things respond to
, signals that come from within or from the environment. Most organisms maintain a stable internal environment and the process by which they do this is
. Living things can also change over time, or
.