In this paper, we explore the critical practice of making sense of students' mathematical ideas. We extend previous research by studying stances prospective teachers adopt, the extent or depth to which they do so, and the types of prospective teachers making sense of students' mathematical ideas. Analyzing the responses of 123 prospective teachers to students' different ideas on an ambiguous mathematical task, our study identifies various stances--descriptive, evaluative, comparative, interpretive, inquiry-based, connective, and projective--and explores the complexity of attributing value, meaning, and significance to student ideas. Our findings offer insights into various types of making sense of students' ideas and suggest that different kinds of attributions are at play for the purposes of observation, assessment, understanding and projection/prediction.