The Metaphor reference article from the Simple Wikipedia on 01-May-2004
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Metaphor

Metaphor is language used to compare without using "like" or "as" - metaphor implies a simile, or a more basic comparison which is called a conceptual metaphor.

A metaphor usually relies on the verb "to be" in some way, so it says "love is war" for example rather than say that a writer sees "love as war", which s/he would do in a simile.

Poetry includes much metaphor, usually more than prose.

Idioms include metaphors, or are metaphors, for example, a shit sandwich is not something you actually would expect to eat, but, it might be something unpleasant you might have to do. The idea that doing a task might be like eating a sandwich is a metaphor. Because it is not commonly used, it is probably not a conceptual metaphor, unless you are in a really bad life situation. If you were, you might make more metaphors that imply you are surrounded by shit and it is getting into everything.

Spam is an example that any email user knows about - this word was originally a metaphor, named after something bad that you might really get served in a sandwich, but would never want. Servers putting unwanted email in your inbox was "like" waiters putting unwanted spam in your food. This was originally suggested by a Monty Python skit.