This document discusses lean prototyping for startups. It defines a prototype as an early version of an idea that is testable. Prototyping is recommended to answer questions, validate assumptions, compare alternatives, visualize ideas, and fail early. Good prototypes are quick to create, cheap, minimal, testable, and measurable. The document outlines a process for rapid and cheap prototyping without coding using tools like Keynote, including planning, building screens and interactions, testing with users, refining, and sharing. It promotes not getting attached to early ideas, prioritizing experiments over fantasies, continuous improvement over perfection, and using prototypes to tell stories.