Summary: In mathematics, students should represent, model, and work with such real-world situations as those found in the physical world, the public policy realm, and society. Additionally, students need to make decisions and be flexible enough to improve their decisions after analyzing realistic situations. The LEGO Pets activity does just that. Modeling with the Common Core’s Standards for Mathematical Practice is a process that begins with identifying variables, analyzing relationships, and formulating a model, then moves into interpreting, validating, and reporting on the results. To bring this modeling process to life in classrooms, teachers need sources of authentic problems. But how can teachers determine the authenticity of a problem that will allow students to model and analyze a situation mathematically? In this article, the authors describe how they developed the LEGO Pets activity (activity sheet included at the end of the article) in which students model a real-world problem situation, first concretely and then abstractly. They taught the activity in three sixth-grade classes in an affluent suburban district as well as in one sixth-grade and one seventh-grade class in a much less affluent suburban district. Approximately thirty students were in each class. The students in all five classes were motivated, interested, and able to work through all the questions posed. Sometimes they needed no scaffolding to answer the questions; at other times, minimal scaffolding; and a few times, extensive scaffolding. (ERIC)