An organism is any living thing that has an organized structure, can react to stimuli, reproduce, grow, adapt, and maintain homeostasis. All organisms are composed of cells, which are the minimal functional unit of life. Organisms include multicellular animals, plants, and fungi, as well as unicellular microorganisms such as protists, bacteria, and archaea. They are capable of reproduction, growth and development, maintenance, and some degree of response to stimuli. Most multicellular organisms differentiate into specialized tissues and organs during their development. Living things need air, water, and food to stay alive.