During lesson study teachers collaborate to design, teach, observe, and reflect on a lesson with the overarching goals of improving instruction and increasing student learning. This article examines how a 6th grade mathematics lesson study project challenged teachers' ideas and assumptions about their students' understanding of fraction concepts. Significant misconceptions were revealed during the teaching of the lesson, prompting teacher learning and changes to instruction. The results highlight critical turning points during the process where evidence collected during lesson observations revealed student struggles and changed how teachers supported their students' thinking moving forward. This article also documents one teacher's journey as she recognizes the need for visual modeling and justification of math concepts to build deeper understanding. There are many different ways lesson study has been implemented in the United States, yet in many ways the effectiveness of lesson study remains uncertain. This paper focuses on key points during the lesson study process that can lead to the development of knowledge and practice. [The issue number on the .pdf (2) is incorrect. The correct issue number is n3.]