*When we interact with a product, we need to figure out how to work it. This means discovering what it does, how it works, and what operations are possible: discoverability. Discoverability results from appropriate application of five fundamental psychological concepts covered in the next few chapters: affordances, signifiers, constraints, mappings, and feedback. But there is a sixth principle, perhaps most important of all: the conceptual model of the system. It is the conceptual model that provides true understanding.*

Design of Everyday Things Revised, Don Norman