The authors believe teaching for understanding begins with the development of a few essential orientations. Teachers must have an appreciation for student centered mathematics teaching, valuing an approach that builds on student thinking. In addition, teachers must appreciate the complexity of students' mathematical thinking and ideas. Once these orientations are in place teachers can attend to student thinking in ways that draw inferences about their understanding (Carpenter, Fennema, Peterson & Carey, 1988; Ma, 1999) and use those inferences to further probe, uncover and extend the complexities of student thinking. The authors contend this is possible through the value of "why." Asking "why?" when teaching mathematics has benefits for both students and teachers. Asking "why?" when teaching mathematics has benefits for both students and teachers. The benefits for students have been highlighted by several authors (Hufferd-Ackles, Fuson, & Sherin 2004; Lampert 2001; Stein, Engle, Smith, & Hughes 2008; Stein, Smith, Henningsen, & Silver 2000). Among others, Hiebert and colleagues (1997) discuss how through the verbalization of their thinking students reach deeper understanding of the mathematical ideas they are working with. When students are asked to reflect on their thinking they form new relationships among mathematical ideas and check old ones. In addition, when student thinking is made visible in the classroom, opportunities are created for students to challenge each other's ideas and ask for clarification. In this paper, the authors illustrate these benefits through a classroom example.