How can you put structures in place in your classroom that build a culture of learning together, making and critiquing arguments and persisting in problem solving when students have varied math backgrounds? You can see how one ninth grade geometry teacher in Brooklyn, NY does it in this WGBH Education video that shows how you might differentiate instruction while encouraging argumentation and critique, supporting the Common Core Standard for Mathematical Practice. From the collection Making the Case, this video highlights teacher Melanie Smith’s 9^th grade geometry class at the Urban Assembly School for Law and Justice in Brooklyn, NY, where students have varied math backgrounds and ambitions.