Behavior refers to the movement of some part of an organism that changes some aspect of the environment. Often, the term behavior refers to a class of responses that share physical dimensions or functions, and in that case a "response" is a single instance of that behavior. If a group of responses have the same function, this group may be called a response class. "Repertoire" refers to the various responses available to an individual; the term may refer to responses that are relevant to a particular situation, or it may refer to everything a person can do.