This document discusses prototyping as a design philosophy. It provides guiding principles for prototyping, including knowing your audience and intent, planning a little and prototyping the rest, setting expectations, prototyping only what is needed, and prototyping early and often. It also discusses different prototyping techniques like paper prototyping and their pros and cons. The document encourages the reader to create a prototype for a geotagging photo sharing application and provides criteria to include in the prototype.