There is considerable agreement about the fact that the presence of researchers in the classroom mediates teaching and learning. Why should two very different forms of human activity, one designed to study the other, interact and mediate each other? In this article, we propose cultural-historical activity theory as a framework for understanding the opportunities that arise for students and teachers from the presence of researchers in the classroom. From the perspective of cultural-historical activity theory and the concept of expansive learning, we articulate the significance of making use of the affordances that arise with the presence of researchers in educational settings. We analyze three vignettes from our research in elementary mathematics classrooms for the purpose of illustrating a cultural-historical activity theoretic explanation of the interaction. We conclude by suggesting that the "impact" of research can be increased at least locally when participants capitalize on the opportunities that arise for teaching and learning when researchers are present.