Important science phenomena--such as atomic structure, evolution, and climate change--are often hard to observe directly. That's why an important scientific practice is to use scientific models to represent one's current understanding of a system. Using models has been included as an essential science and engineering practice in the "Next Generation Science Standards" (NGSS Lead States 2013). This article describes an instructional unit that uses an iterative process of generating, critiquing, and revising complex scientific models that address sea level rise.