Entrepreneurship education is increasingly shifting from learning about entrepreneurship to developing entrepreneurial skills. One skill all entrepreneurship students need to develop is the ability to assess whether or not there is a market for their new product or service. Recent popular trade books like Eric Ries's "Lean Startup" and Ash Maurya's "Running Lean" provide insights into how to capture validated learning to determine what people really want. This experiential classroom exercise allows entrepreneurship students ranging from undergraduates to executive MBA candidates to go through an entire cycle of Ries's build-measure-learn framework that is central to the lean startup. This exercise consists of five modules--customer persona, value proposition canvas, experiment map, minimum viable product, and running the experiment--that can be completed over five lecture sessions. Students learn how to "run lean," determine the market for a new product or service, and apply these skills later in new ventures or established firms.