Fragile understanding is where new learning begins. Students' understanding of new concepts is often shaky at first, when they have only had limited experiences with or single viewpoints on an idea. This is not inherently bad. Despite teachers' best efforts, students' tenuous grasp of mathematics concepts often falters with time or when presented within a slightly different context. How can teachers help students develop a robust, multifaceted, interconnected, and lasting understanding of mathematical concepts? Rather than avoiding students' fragile understanding and misconceptions, Kara Suzuka and Linda Venenciano have come to embrace them as a natural part of learning, seeking ways to harness them to build stronger understanding. Toward this end, they use the framework described in the National Council of Teachers of Mathematics' (NCTM's) "Putting Essential Understandings into Practice: Geometry, 9-12" to engage students in three processes: (1) reversibility; (2) flexibility; and (3) generalization (RFG). In this article, Suzuka and Venenciano define these processes and the types of questions associated with each one. They go on to show how the RFG questioning framework serves as a powerful tool for teachers, allowing them to create mathematical problems that reveal fragilities in students' current understanding as well as offer rich contexts for students to make connections, see new applications, and add to their conceptual understanding.