A demandance is a psychological "pull" exerted by a stimulus. It is closely related to the theory of "affordance". I introduce the theory of demandance, offer some motivating examples, briefly explore its psychological basis, and examine some implications of the theory. I exemplify some of the positive and negative implications of demandances for design, with special attention to young children and the design of educational products and practices. I suggest that demandance offers an approach to one of the persistent mysteries of the theory of affordance, specifically: Given that there may be many affordances in any particular setting, how do we choose which to actually act upon?