We explore the role that metaphor plays in developing expressive devices by examining the MetaMuse system. MetaMuse is a prop-based system that uses the metaphor of rainfall to make the process of granular synthesis understandable. We discuss MetaMuse within a framework we call ”transparency” that can be used as a predictor of the expressivity of musical devices. Metaphor depends on a literature,or cultural basis, which forms the basis for making transparent device mappings. In this context we evaluate the effect of metaphor in the MetaMuse system.