While computing technologies are widely available in secondary schools, these technologies have had only limited impact on changing classroom practices. Partly, this can be attributed to an underdeveloped understanding of the role of the teacher in engaging in classroom practices that can support student learning with technology. In this study, we analyzed the teaching practices that supported students' learning of a conceptually rich and deep topic (the average rate of change) when using an exploratory computer simulation environment. The results illustrate the demands placed on teachers when faced with the multiplicity of student ideas generated by their interactions with the simulation and three aspects of a teaching practice in response to those demands. These findings contribute to evolving frameworks for understanding meaningful and productive technology use in teaching secondary mathematics. [For the complete proceedings, see ED584829.]