Quick-and-dirty prototyping is the deliberate practice of building the lowest possible fidelity prototype that can still generate useful feedback, using whatever materials are at hand, in the shortest possible time, without any concern for visual quality or production standards. The 'dirty' is intentional: the rough, unfinished quality of the prototype signals clearly to users that nothing is decided, inviting honest critical feedback rather than polite approval of what appears to be a finished design. It is the antidote to the over-investment that slows most design processes.