This article draws on Lampert’s three-pronged model of teaching practice (Lampert, 2001) to explore the possible contributions of elementary classroom teachers to the learning-to-teach-mathematics experiences of preservice teachers (PSTs). The authors focus on a third-space context in which mentor teachers (MTs), PSTs, and university teacher educators collaborated to plan and analyze task-based problem-solving interviews of children. The authors analyzed the MT's contributions to a third-space activity involving the task-based interview. This analysis also suggests ways in which university teacher educators might enhance the development of methods/field third spaces by anticipating and preparing to leverage MT contributions.