Positive reinforcement occurs when a reward, sometimes called a reinforcer, is given for a specific desired behavior. It can be a tangible object, an activity, something sensory, or social. If the behavior increases or is maintained in order to receive the desired stimulus, then it is a positive reinforcer. Pain is a subjective experience, and no objective tests exist to measure it. Pain assessment in patients who are unable to verbally communicate with medical staff is a challenging problem.