Mock-ups are used mainly to acquire feedback from users about designs and design ideas early in the design process. Mock-ups are 'very early prototypes' made of cardboard or otherwise low-fidelity materials. The user, aided by the designer, may test the mock-up (imagining that it works) and thus provide valuable feedback about functionality/usability/understanding of the basic design idea/etc. Mock-ups make it possible to do usability testing early in the development process.