Building apps is a costly endeavor, with many potential pitfalls and points of failure. Getting user and stakeholder feedback early can save your team time and effort, particularly as the product vision develops. We will demonstrate that it is never too soon in the development cycle to start building prototypes. Building your first prototype is simple, often requiring nothing more than paper and pencil! More generally, we will highlight how prototyping might be used in open-source communities, particularly as a distributed collaborative exercise. The iterative and interactive aspects of the open-source case are widely relevant to successful prototyping among diverse product teams.