The Why2-Atlas system teaches qualitative physics by having students write paragraph-long explanations of simple mechanical phenomena. The tutor uses deep syntactic analysis and abductive theorem proving to convert the student’s essay to a proof. The proof formalizes not only what was said, but the likely beliefs behind what was said. This allows the tutor to uncover misconceptions as well as to detect missing correct parts of the explanation. If the tutor finds such a flaw in the essay, it conducts a dialogue intended to remedy the missing or misconceived beliefs, then asks the student to correct the essay. It often takes several iterations of essay correction and dialogue to get the student to produce an acceptable explanation. Pilot subjects have been run, and an evaluation is in progress. After explaining the research questions that the system addresses, the bulk of the paper describes the system’s architecture and operation.