Thoughts conveyed through gesture often differ from thoughts conveyed through speech. In this article, a model of the sources and consequences of such gesture-speech mismatches and their role during transitional periods in the acquisition of concepts is proposed. The model makes 2 major claims: (a) The transitional state is the source of gesture-speech mismatch. In gesture-speech mismatch, 2 beliefs are simultaneously expressed on the same problem--one in gesture and another in speech. This simultaneous activation of multiple beliefs characterizes the transitional knowledge state and creates gesture-speech mismatch. (b) Gesture-speech mismatch signals to the social world that a child is in a transitional state and is ready to learn. The child's spontaneous gestures index the zone of proximal development, thus providing a mechanism by which adults can calibrate their input to that child's level of understanding.