The Common Core State Standards for Mathematics (CCSSM) (CCSSI 2010) outlines ambitious goals for fraction learning, starting in third grade, that include the use of the number line model. Understanding and constructing fractions on a number line are particularly complex tasks. The current work of the authors centers on ways to successfully introduce the number line to third-grade students and how working with this challenging model helps them develop increasingly robust understandings of fractions. In this article, they share their work with children and teachers from third-grade classrooms in an urban district in the upper Midwest. In phase 1 of their work, they uncovered student misunderstandings, and they share examples of typical errors among these third graders. They then give examples of their curriculum intervention, designed to attend to these misunderstandings and implemented in phase 2. The authors conclude by sharing examples of successful third graders' thinking about fractions on the number line.