" Other clues to how things work come from their visible structure - in particular from affordances, constraints, and mappings. Consider a pair of scissors: if you have never seen or used them before, you can see that the number of possible actions is limited. The holes are clearly there to put something into, and the only logical things that will fit are fingers. The holes are affordances: they allow the fingers to be inserted. The sizes of the holes provide constraints to limit the possible fingers: the big hole suggests several fingers, the small hole only one. The mapping between holes and fingers - the set of possible operations - is suggested and constrained by the holes. "

― Don Norman, , The Design of Everyday Things