Cathy’s lesson is posed as a “real-world” situation, which motivates a context for her students’ investigations. There are many truly real-world applications of mathematics (e.g., the mathematics of global warming). These applications are especially important for students to experience in order to understand the relevance and importance of mathematics in their lives. During these two days of geometry investigations, Cathy’s students work individually and in small groups to develop convincing arguments (for themselves, for a partner, for a skeptic) about the properties of quadrilaterals. Throughout their conversations, her students need to make use of tools strategically. Knowing how to use a tool is very different from knowing when to use it, and students need a lot more practice with problems that require them to dig deep to find, from all that they know, the tools and ideas that will help them solve a new and unexpected problem.