It is becoming increasingly important for students to go beyond engaging in design by reflecting upon the nature of engineering (NOE) and wrestling with questions such as, "What do engineers do?" and "What is engineering?" This article describes an activity in which middle school students are asked to create a product to help a fictional bank sort coins. This design activity can be used to meet Next Generation Science Standards (NGSS) middle school engineering standards three and four (MS-ETS1-3, MS-ETS1-4) that target optimization and the iterative nature of engineering. Over the course of three 45-minute class periods, students identify the criteria and constraints of the problem, create a model, and then gather and analyze data to optimize their coin sorters. Teachers ask questions throughout this process to prompt students to reflect on their in-class experiences with the coin sorters and explore how those experiences might relate to what engineers really do. This explicit and reflective approach of planning representative activities and asking reflective questions demonstrably helps students deepen their understanding of the nature of science, and the authors have found it applies equally to teaching NOE.