This study investigated how six Year Four (9-10 years old) students interacted with a dynamic (zoomable) digital number line to demonstrate knowledge and understandings of decimal fractions. Results from a task-based interview indicated that the zoomable number line is able to assist students in developing conceptual understandings of decimal density, place value and relative size. The app proved to be a powerful mathematical representation of decimal fractions because of its dynamic affordances that allowed the students to 'see' concepts that would otherwise be 'unseeable' when using a traditional static number line.