This article describes a teaching experiment conducted in a seventh-grade classroom. It explores ways that students used four types of problem-solving knowledge through writing their responses to algebraic problems. The problems were presented to students in the geometric contexts of growth and change and size and shape. It was found that writing gave students the opportunity to create their own detailed problem-solving knowledge by constructing it for themselves, not by having the teacher tell them the procedures. They participated in the process of "knowledge-getting," which helped them to learn to include the important steps so that there were no gaps in their reasoning. Through this experience, students learned to communicate their knowledge by explaining and justifying their solutions. While describing the generalizable patterns of the relationships between the quantities in the problems, they made their algebraic thinking explicit. This explicitness helped them develop the knowledge needed to solve the algebraic problems. (Contains 13 figures.)