From small interactions to full page designs, the best place to try out responsive ideas is in the browser.
A collage of original but disparate design elements meant to demonstrate ideas without committing to a particular layout. As originator Dan Mall put it:
A full comp often requires ideas to be fully realized. An element collage allows me to document a thought at any state of realization and move on to the next.
A collage of images, textures, typography, colors or other materials, intended to evoke or visually summarize a particular style or concept.