Many times teachers hear students say: "Why are we learning this? Why do we have to know this? When are we going to use this outside of class time?" These common questions are probably familiar to most high school teachers. An 11th-grade English teacher attended a university-school district professional development (PD) program on interdisciplinary mathematics to gain insight on how to answer these students' questions and develop a new perspective on implementing "interdisciplinary"--also called "integrative"--curriculum. When applying PD program concepts, the English teacher integrated solid and plane geometry in an interdisciplinary manner during a study of "The Scarlet Letter" (2004) by Nathaniel Hawthorne in his self-contained 11th-grade class. This analysis of his strategy provides supporting research, summarizes an interdisciplinary approach for classroom use, integrates an assessment strategy, and generalizes the approach for multiple interdisciplinary projects. (Contains 2 tables and 2 figures.)