This case study takes the form of a dialogue between three students searching for a definition of life. The characters are very loosely based on Galileo’s Dialogue Concerning the Two Chief World Systems. The three students are a physics student (Dave/Salviati), a biology student (Mary/Sagredo), and a chemistry student (John/Simplicio). The case consists of two parts. The first part examines the need for a definition of life, and the two most common approaches: (1) define life by listing the properties of life as we know it (e.g., organization, metabolism, homeostasis, growth, reproduction, response to stimuli, and evolution), and (2) the Joyce definition that NASA uses, i.e., life is a self-sustained chemical system capable of undergoing Darwinian evolution. The second part of the case study discusses the approach of Schrödinger: life is defined by physical principles, specifically those of entropy and free energy. The case was designed for an introductory course on astrobiology, but it can also be used in any course that discusses thermodynamics (specifically, the second law of thermodynamics) or the definition of life (e.g., biology courses).