The inertial balance is one device that can help students to quantify the quality of inertia--a body's resistance to a change in movement--in more generally understood terms of mass. In this hands-on activity, students use the inertial balance to develop a more quantitative idea of what mass means in an inertial sense. The activity also helps students refine their ability to use numerical data and line graphs as a tool for making predictions. (Contains 4 figures. Appended to this document is an activity sheet, accompanied by instructions for interpreting the data presented in graph form.)