If you’ve ever followed a recipe, you know that the amount of each ingredient and the order in which you mix them matters. Chemical engineers use these same principles when designing processes. Students will learn about a girl who designs a better way to make play dough. The activities in this unit reinforce the science concepts “solid” and “liquid” as students explore the properties of different materials—and the properties of mixtures of materials. The final engineering design challenge? Design a process for making high-quality play dough.