Writing allows students to clarify their thinking. Through the process of writing, students recognize what they know and what they still need to learn. This article describes how a middle school mathematics teacher and language arts teacher worked with a secondary content area literacy professor during a specific writing assignment. The "How-to" book writing project was designed to help 124 sixth-grade students deepen their understanding of mathematics and share their mathematical expertise. (Contains 4 figures and 1 table.)