This chapter addresses experimental and field observations, which include aspects of Asking Questions and Defining Problems; Planning and Carrying Out Investigations; Constructing Explanations and Designing Solutions; and Engaging in Argument From Evidence. The chapter focuses on one portion of a scientific investigation—the controlled experiment. Controlled experiments involve making measurements under conditions in which cause and effect can be established. This is harder to do than merely stating the nature of that relationship or making qualitative observations under less controlled conditions. In this chapter, the experimental activity measuring solubility of water vapor in air has the primary goal of explaining the water cycle, a part of how the Earth works. A materials list, teacher background information, safety practices, and outcomes are provided. This book selection includes the Table of Contents, Acknowledgments, About the Authors, About the Book, Introduction, and Index.