Mathematical modeling involves using mathematics to represent, analyze, and make predictions or decisions about real- world situations. Garfunkel and Montgomery (2016) elaborate on six components of the mathematical modeling process, including identifying the problem, making assumptions and identifying variables, doing the math, analyzing and assessing the solution, iterating, and implementing the model. The modeling process begins with connections to a real- world context and students' experiences. These real- world contexts could be in the classroom, school, or broader community. Next, students make sense of a situation within the context, sharing what they notice, understand, and wonder. As students share questions about the situation, teachers support them in posing an authentic and relevant modeling problem. Once students pose the problem, students identify and list important quantities and assumptions. Generally, students make a list of things they already know, things they need to know, and things they can assume or decide as a team. Students represent and explore different ways to relate the quantities and assumptions identified using pictures, equations, tables, and other mathematical representations. Eventually, students create a modeling solution that answers the problem they posed. Finally, students interpret their models by comparing them to those of other groups and assessing their solutions. This article describes how one teacher uses mathematical modeling to turn an authentic context in her school into a mathematically rich task. Students drive the work as they collect and analyze data, predict future outcomes, and take action to reduce plastic water bottle waste at their school.