In this article, the author asserts that asking and responding to diagnostic questions is the single most important part of teaching secondary school mathematics. He notes the importance of formative assessment and recommends a formative assessment strategy that requires students to be public about their answers to questions, displaying their thoughts in front of their teachers and peers in the moment. This can be accomplished by ensuring that the questions teachers ask are seen as tools of learning rather than tools of assessment, with no negative consequences for being wrong. Diagnostic questions are designed to help identify and understand students' mistakes and misconceptions in an efficient and accurate manner. A good diagnostic question (1) should be clear and unambiguous; (2) should test a single skill or concept; (3) should allow students to answer in fewer than 10 seconds; (4) should reveal specific misconceptions; (5) and cannot be answered correctly by the student while still holding a key misconception.