In this study, we examined prospective middle school mathematics teachers' reflective thinking skills to understand how they learned from their own teaching practice when engaging in a modified lesson study experience. Our goal was to identify variations among prospective teachers' descriptions of students' thinking and frequency of their interpretations about how teaching affected their students' learning. Thirty-three participants responded to open-ended questionnaires or interviews that elicited reflections on their own teaching practice. Prospective teachers used two forms of nuance when describing their students' thinking: (1) identifying students' specific mathematical understandings rather than general claims and (2) differentiating between individual students' thinking rather than characterizing students as a collective group. Participants who described their students' thinking with nuance were more likely to interpret their teaching by posing multiple hypotheses with regard to how their instruction affected their students' learning. Implications for supporting continued growth in reflective thinking skills are discussed in relation to these results.