In this study, we examine students’ mathematical reasoning within a technological environment designed to support understanding of relationships between quantities with adjustable measuring units. In particular, we provide a cross-sectional snapshot of how 30 elementary students (Grades 3–5) engaged in a series of fraction-as-measurement tasks using a “Dynamic Ruler” that could continuously dilate unit sizes. Screencast recordings were collected from a task-based clinical interview and analyzed to investigate children’s mathematical actions and mathematical ideas. Students’ reasoning patterns were characterized using four distinct types (low attending, holistic estimating, determining, and commeasuring) based on their solution strategies. Findings suggest that the Dynamic Ruler tool can elicit rich conceptions of fractions and even prompt novel approaches such as commeasurement. We conclude by drawing insights into how elementary students might use dynamic technology meaningfully.