An assessment question involving Newton’s 2nd law was administered in a physics course for preservice elementary teachers before and again after instruction. The posttest included a prompt asking students to describe the specific ways their thinking changed. Student reasoning was coded for physics content accuracy; many students exhibited changes from primitive, experientially-based reasoning to more formal reasoning. Students' self-reported reflections were then compared to the differences in the pre- and posttest codes. We find that many students do not identify substantive changes in their reasoning, while other students reflect at only a surface level. We also find that some students overestimate their initial level of understanding.