Teaching mathematics is a difficult and complicated task. For student teachers, who are extremely new to the mathematics classroom, this difficulty is magnified. One of the biggest challenges for student teachers is learning how to effectively use the student thinking that emerges during mathematics lessons. I report the results of a case study of two mathematics education student teachers. I focus on how they make decisions while teaching in order to use their students' mathematical thinking. I also present analysis of the student teachers' discourse patterns, the reasons they gave to justify these patterns, and how their reasons affected how they used their students' thinking. I found that generally the student teachers used student thinking in ineffective ways. However, the reasons the student teachers gave for using student thinking always showed the best of intentions. Though given with the best of intentions, most of the reasons for using student thinking given by the student teachers were correlated with the student teachers ineffectively using their student's thinking. However, some of the reasons given by the STs for using student thinking seemed to help the student teachers more effectively use their students' thinking. I conclude with implications for preparing future student teachers to better use student thinking.