Learning based upon the consequences of behavior. For example, a rat may learn to press a lever when this action produces food. Instrumental or operant behavior is the behavior by which an organism changes its environment. The particular instances of behavior that produce consequences are called responses. Classes of responses having characteristic consequences are called operant classes; responses in an operant class operate on (act upon) the environment. For example, a rat's lever-press responses may include pressing with the left or with the right paw, sitting on the lever, and so on, but all these taken together constitute the operant class called lever pressing.