Many middle school students still struggle with the concept of density, without which it is difficult to understand overturning circulation. This chapter begins with physical models of temperature and saline density differences and moves to looking at the work of a researcher building computer models of overturning circulation. This is a timely topic because scientists are trying to find out if warming temperatures could set off a feedback loop in which the ocean releases carbon dioxide, or doesn’t absorb it as well, leading to more warming. The reading strategy featured here is “previewing diagrams and illustrations.” A materials list, teacher background information, references, assessment, and outcomes are provided. This book selection includes the Table of Contents, Acknowledgments, About the Author, Image Credits, and Index.