Context for the Learning Progressions Work Before describing the dimensions of our learning progression, we first provide a brief description of thecontexts of our work. Our research has taken place in several elementary and middle school classrooms. Illustrations in this article come from 5th and 6th grade classrooms. In the elementary level, we designed a 6-week unit for 5th grade students around modeling evaporationand condensation phenomena, using a solar still as an anchoring phenomenon.A solar still is a device that can distill or purify water using the sun as a heat source. In the device, water evaporates into the air and condenses on a surface from which it is collected and used as clean drinking water. The unit was designed to engage students in the modeling practices of constructing, evaluating, revising, and using scientific models of evaporation and condensation phenomena to explain how the solar still works and how other evaporation and condensation phenomena occur (see Table 2 for an outline of our general instructional sequence). In particular, students construct and revise a model of evaporation and, later, a model of condensation based on empirical evidence of the presence of water vapor in the air. They use newly introduced ideas of water as being composed of smaller bits or particles that spread out in the air so that they cannot be seen (evaporation) and clump together into larger bits of water drops (condensation) under particular conditions. Students’ expressed models take the form of written diagrams. The modeling practices within the unit are infused with metamodeling conversations at key moments when epistemic issues are the most relevant (e.g., discussing the evaluation ofmodels when comparing and contrasting different models for the process).