We have replaced the input understanding component of CIRCSIM-Tutor, an intelligent tutoring system that engages the student in Socratic dialogue. Students type free-text answers to the computer’s questions. Even though the questions can be answered very simply, the variety of student responses prompted us to make the understanding component more robust. The new software also enables the tutor to recognize and classify a greater number of unexpected responses. In this paper we report on the motivation and design of the new software and our results in using it.