Basic Considerations for Metaphors
A metaphor should be quite common and familiar to users. An obscure metaphor will only serve to confuse, not assist a user
A metaphor should have a single primary implication, rather than requiring the user to choose among two or more possible interpretations
A metaphor should be naturally, logically related to the activity to be performed on the computer
A metaphor should have as its purpose clarification and facilitation of the human-computer interface; it should never be employed simply because it is cute or clever.
Theoretical Foundations of Multimedia Chapter 7