Formative assessment probes are effective tools for uncovering students' ideas about the various concepts they encounter when learning science. They are used to build a bridge from where the student is in his or her thinking to where he or she needs to be in order to construct and understand the scientific explanation for observed phenomena. In the process of using formative assessment, the function of teaching and learning shifts from one where children are merely provided with information about new concepts by the teacher to one where children have their own theories, which are challenged and reconstructed as new evidence is assimilated. To make this shift, teachers must first use formative assessment to find out what students' initial ideas are prior to instruction--as this is the conceptual framework the student will use to make sense of the new concepts and experiences they encounter in the science classroom. Second, this information needs to be used by the teacher to present new concepts or design experiences in a way that the student will be able to assimilate into their existing conceptual framework and recognize when their own ideas need to change. While this constructivist view of teaching and learning works well with children, it also supports teacher learning. This column explores how the formative assessment probe "Is It Melting?" was used to confront teachers with their own alternative conceptions and help them reconstruct their ideas as they were presented with evidence that did not fit their initial conceptions.