The "Next Generation Science Standards" ("NGSS") and the guiding "Framework" invite students to engage in practices of scientific argumentation and modeling through which they can collaborate as young scientists and develop understandings of disciplinary core ideas and crosscutting concepts. This article shares instructional tools and practices developed in a second-grade classroom that enabled students to build explanations through argumentation and modeling over the course of an instructional unit. While the authors narrate how the unit unfolded and unpack strategies that supported students' engagement in these practices, particular attention is given to an important tool--the Agree/Disagree T-chart--that grounded class conversations over time. The Agree/Disagree T-chart can support students in (1) generating and refining claims about a phenomenon; (2) rallying evidence from different unit activities; (3) understanding how arguments and models co-develop over time, as new evidence is integrated; and (4) making their reasoning public and developing a community that values learning together. The article describes how Ms. V, a second-grade teacher who helped develop the Agree/Disagree T-chart tool and instructional unit, used the T-charts and other tools/strategies to support students' argumentation and modeling in the beginning, middle, and end phases of the unit. While the unit was mostly taught during time allotted for science, some reading and writing activities were incorporated into literacy lessons when the activities could support multiple standards across disciplines.