This document discusses prototyping and its role in the innovation process. It describes how the Copenhagen Institute of Interaction Design uses prototyping to gain insights into what is not yet known, rather than waiting until enough is known. Different levels of prototype fidelity are explored, from low-fidelity paper prototypes to validate concepts to mixed and high fidelity prototypes. The value of 3D printing for enabling experience prototyping is highlighted through several case studies. The document argues that prototypes help resolve issues with the "fourth dimension" of experience.