Hopscotch, basketball, or hide-and-seek? Children have many choices at recess, and while making these choices they must consider and make trade-offs. The way they make these decisions is not that different from the thought processes engineers use when making design trade-offs. Engineers have to make trade-offs because a design that meets all requirements of a problem equally and perfectly is impossible. Hence, engineers talk about optimal solutions--not perfect solutions. These necessary trade-offs make design decisions challenging and differentiate engineering practices from scientific practices. This article reports on an engineering optimization egg-packaging activity for fourth and fifth grade students. As students worked through the egg-packaging design project they recorded their design sketches, data, and reflection pieces in an engineering journal. Students learned that engineers rely on their knowledge of science and mathematics as well as specialized skills such as design, optimization, and making trade-offs. An important aspect of this activity was helping students identify both successes and failures in their designs. The students learned that something does not have to be perfect to still work well, that most designs have aspects that work well and other aspects that could be improved, and that the best designs are a combination of many ideas.