Formative assessments serve to engage students in active sense-making, making them critical tools for both teachers and students. Though formative assessments may be an informal mode of assessing, they are one of the most important, especially when teaching science. As a subject, teaching science does not have the same kind of beginning-middle-end flow that teaching language arts or math can because investigations and explanation-building are often ongoing, multiyear efforts to understand complex phenomena. Effective formative assessments allow a teacher to know where students are in terms of understanding said phenomena, and they act as guides for planning lessons that flow and build on students' efforts to construct explanations. This article describes how teachers can use first graders' scientific models as formative assessments. The authors co-planned a unit around the "Next Generation Science Standards" ("NGSS") space systems performance expectations for first grade. In this article, the authors provide a brief overview of the unit, share ideas about how they developed and used formative assessments throughout the unit, and then offer a closer look at one lesson and its formative assessments.