In treating design as a language, we can say that the shape of a form says something about its behaviour. Thus:

• An idiom is a unique shape. Its behaviour is idiosyncratic must be explicitly tested and learned.
• An analogue may have a familiar shape and may behave similarly, but along a different dimension.
• A metaphor may resemble something familiar but in fact represent something completely different.

The shape of the sundial is idiomatic — it's its own thing and has to be learned independently. Likewise with the digital clock, or rather the notation of time it depicts. It is a descendant of the mechanical, or analogue clock, which is itself an analogue of the sundial. The speedometer is an analogue of the mechanical clock, except it depicts speed instead of time. The wristwatch icon — the one that appears when a typical computer is busy — is a metaphor. You can't tell time with it, it simply shows up when you have to wait.

Although a metaphor is usually immediately understood, it brings its limitations along with it. Metaphors in design break down when the form's underlying behaviour and the metaphor part ways. Susan Kare's wristwatch is appropriate for its small but important role in PC desktop-metaphor GUIs, but the astute reader will acknowledge that it only exists because real desks aren't affected by the computational complexity of the work performed upon them.