Teachers across the nation are taking up the call to prepare students for new standards and assessments that require written explanations on a variety of math tasks. But what does it really mean to "explain your answer," and how do educators begin supporting students to write in a genre that is new to both students and teachers? This article describes insights from a yearlong project conducted by a researcher and three teachers from different school districts. The project team met once a month for three-hour workshops in which we discussed mathematical explanations and wrote some of our own. The authors designed five lessons to support students in writing rich mathematical explanations, and the teachers piloted these lessons in their classrooms. In a process similar to lesson study, they videotaped the lessons, reflected on what worked and what did not work, and analyzed students' writing to give information about what next steps to take. In this article, the authors share their first project-developed lesson in which a collaborative writing activity sparked powerful class discussions about what makes a good explanation. They include examples from two fifth-grade classes and offer suggestions to other teachers who want to engage their students in collaborative mathematical writing.