Lesson study is a professional development activity that increases teachers' attention to student thinking. However, coordinating teachers' live observations of a lesson can be challenging. Using the framework of "distributed cognition," we investigate whether technology supports teachers' examination of student thinking and aids the processes of developing and archiving new knowledge about students' mathematical understandings in lesson study. We use data from a professional development intervention where four geometry teachers discussed videos and animations within a lesson study cycle. We ask, (1) "what modifications to the lesson did the teachers offer," (2) "what were the justifications and sources (i.e., videos, animations, or teacher lesson observations) of those modifications," and (3) "which suggested modifications were implemented in the revised lessons?" Using a modified Toulmin model for arguments, we analyzed the teachers' arguments about lesson revisions. Findings show that the teachers adapted the lesson based on their discussions of videos and animations. The study shows a case where using technological resources promoted teacher learning by supporting teacher noticing of student thinking and eliciting knowledge about students that otherwise would be ephemeral. We discuss implications for using technology in professional development.