The role of gestures in communication is still debated: some claim that gestures are merely ancillary forms of expressions, whereas others suggest a central role of gestures in the development of language. This article provides data in support of the overarching hypothesis that gestures have a transitional function between ergotic/epistemic movements of hands and symbolic expressions. The context for the study of these transitions is constituted by school science laboratory activities conducted by students who are also asked to describe and explain while still within proximity of the materials of their investigations. It is hypothesized that communication is distributed across the context (verbal, gestural, material) and shifts increasingly into a verbal modality as students become familiar with the phenomena they are to learn about. Furthermore, it is hypothesized that initial temporal delays between gestures and the corresponding words decrease and finally disappear so that gestural and verbal modalities coincide. It is suggested that engaging in communication in the presence of material has an important cognitive function in that it affords a distribution of cognition across different modalities until individuals have developed the competence to express themselves effectively in the verbal modality.