This book reports on findings from a study regarding two experienced secondary school teachers' approaches to handling unanticipated student questions during typical classroom discussions. Although teachers can anticipate common questions or misconceptions from students when planning a lesson or unit, teachers can never fully prepare for how students construct new knowledge. This book examines a blend of mathematical and pedagogical knowledge teachers used to respond quickly to unanticipated student questions. Additionally, this book illustrates the various patterns teachers exhibited when addressing students' comments or questions within typical classroom discussions.