The definition of “life” is the most controversial subject—just mention the word to scientists would undoubtedly be a heated debate. It affects every branch of biology—from life on Earth to the possibility of life in outer space. But some general, often agreed-upon criteria exist for the definition of life (although some creatures exist that are contrary to the rules): Living organisms are usually complex and highly organized (with exceptions); most creatures respond to external stimuli (for example, plants that recoil on touch, and for higher level organisms, the ability to learn from the stimulus); the majority of organisms try to sustain internal homeostasis (a relative balance of an organism’s internal systems, such as maintaining its temperature); most tend to take their energy from the surrounding environment and use it for their growth and reproduction; and most organisms reproduce (asexually or sexually—or even both), with their off-spring evolving over time. Of course, these definitions do not take into consideration alternate forms of organisms—such as possible extraterrestrial life that could upset our Earth-centric view of life!