Thursday Thoughts - Metaphors and Similes

I'm trying to get some regular posts going here, hence the "Thursday Thoughts" name. Today I'd like to work my way through the idea of when to use metaphors, when to use similes, and what makes them effective. I need to put out the disclaimer however, that I am not classically trained in English literature. I am hopefully not going to be spouting nonsense, but feel free to correct any obvious mistakes.

Simile construction consists of using the word 'like' when comparing two things. An example would be "The tension cut like a knife." While this particular example is fairly cliche, it works well enough to explain how to use a simile. A metaphor however, is a comparison where you do not use the word 'like'. An example of this would be "His car was a bullet, flying down the highway with no regard for obstacles." The difference between the two is this: When using simile, an author is comparing the two concepts or objects side by side. When using metaphor however, one object or concept is being completely overlaid by another, becoming that object figuratively.

The rule-of-thumb is to avoid similes whenever possible. The idea is that a metaphor is much stronger in a literary sense than a simile. It conveys the comparison with greater emphasis, particularly because you are replacing part or all of the original object with a new figurative description. You are adding to things, enhancing them in a way. Similes are straight comparison, and most of the time they impact the reader less. Consider the following example:

"Her hair was like an unkempt haystack."

Or

"Her hair was an unkempt haystack."

While the change is very subtle, the second example doesn't imply that her hair was a haystack; it states it emphatically. There is no possibility that the reader will mistake what her hair looks like, because the sentence describes it in crisp, clear words. This is what we want as authors, after all. We want our readers to completely visualize the story, see it through our eyes in all its splendor.

So when should you use a simile? This is a much harder question for me to answer. The entire time I've been writing this post, I've been trying to come up with a good reason. The only one that comes to mind is if using a simile is necessary in order to stay true to the story. For example, if someone is speaking, describing something, and they don't usually talk in metaphors (i.e. they aren't as intelligent, aren't as verbally proficient, etc), then a simile is necessary. You can even use this idea if the person is just thinking about things, since we often think using the same words we speak in. Other than that, I would advise using a well-written metaphor whenever possible instead of a simile.

So have I missed anything here? Are there other instances you can think of where a simile would be appropriate?