Life is a characteristic that distinguishes physical entities with biological processes from those that do not. It encompasses various forms, including plants, animals, and microorganisms, all of which exhibit growth, reproduction, and response to stimuli. The study of life is known as biology, which explores the complex interactions and functions of living organisms. At its core, life is defined by the presence of cells, the basic units of life. Organisms can be classified as unicellular, like bacteria, or multicellular, like humans and trees. Life also relies on energy, typically obtained from the environment, to sustain its processes and maintain homeostasis.