This paper attempts a unified explanation of such apparently non-utilitarian behaviors as curiosity, manipulation, and exploration as manifestations of "playful behaviors" on the one hand and stereotyped responses on the other. Sensorhesis names the new theory offered to explain the existence and nature of playful behavior. Play and stereotyped behaviors are juxtaposed on the same continuum by virtue of their possible opposite action in maintaining optimal stimulus input in an organism (0). Play generates large information loads by virtue of its elements of novely; stereotyped behavior generates minimal information either as a substitution for stressful stimulus input or a bacuum activity under conditions of perceptual deprivation. The adaptation of the 0 to a given level of stimulus complexity requires an increasingly complex interaction with the environment to maintain the information flow and optimal arousal. (WY)