In this paper we discuss how metaphors, and in general the translation of meaning between different knowledge domains, relate to the understanding of the interface between a human and a computing system, hence to its naturalness. Focusing on tangible user interfaces we analyze metaphors, analogies and symbolic representations developed in the computing interaction area to represent concepts in human computer interfaces along three main dimensions: the coherence of their use in linking concepts of some domain to the implementation of the interface, the coverage of the concepts implemented with respect to the amplitude of the knowledge domain, and their compliance with respect to the expectations of the human experience. We justify the choice of these properties by analyzing five simple tangible systems proposed in the literature according to these three dimensions, trying to understand how they support the development of interaction styles that users can exploit naturally.