In this practitioner-focused article, the authors present a mathematical writing activity for preservice elementary school teachers. The products of this activity, referred to as "journals," are detailed, representation-rich explanations of problems solved in small groups. These journals were originally exchanged across institutions in a traditional peer review process; and the addition of a computer-mediated communication platform transformed the peer reviews into interinstitutional conversations. Analysis of the journals and the conversations they sparked brought to light four opportunities to develop mathematical knowledge for teaching. Evidence in the form of journal and review excerpts suggests that the process, enhanced by the computer-mediated communication, enabled preservice teachers to deepen their understanding of foundational concepts from the elementary school curriculum, to communicate mathematics more effectively to others, to better make sense of the mathematical thinking and writing of others, and to incorporate their colleagues' suggestions into their future writing. Suggestions and considerations for implementing this activity are discussed.