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Churchill, when asked to hurriedly write a short speech, responded by saying, "I can write you a speech in ten minutes. However, if you want a short speech it will take several hours."
The fact is, that so long as you don't mind verbal redundancy built into a speech you can in fact write one very quickly. Although probably full of unnecessary sentences, words and phrases because there will be no time for paring, or editing text.
The idea of Flash Fiction is to make your writing as concise as possible so, in 1,000 words or less, you deliver the reader a completed story.
Using Flash Fiction as a regular exercise not only improves written work (as it compacts it like a 'car crusher'), but also allows you to explore ideas for stories in a way that forces you to consider the start, middle and end without useless bulk and filler.
In Flash Fiction the words you remove are as important as the ones that remain. The story, absent of anything to trip the reader runs like an Express Train. Delivering a fast-paced story; the reader gripped to the very end.
You don't need to set stories to any specific maximum, it's a matter of choice. However, there are many instances where publishers will request a specific word count. So, being able to write to set limits is a professional advantage.
To make Flash Fiction' special requires imagination and hard work. Mostly hard work. A 1,000 word Flash Fiction story will take me a full day, or more, to get 'word perfect'. About the same amount of time it takes me to write 3,000-5,000 words for a novel.
If you think you would like to try Flash fiction then select a story you have written, or an idea for one and write it quickly paying attention to the word count. When you get to 1,000 words, even if you have not finished - Stop!
Go back through what you have written, take out every word that is superfluous. You might remove whole sentences, even paragraphs. If it is not important to the story - remove it. With a little practice you will learn to do this with ease.
When you have completely gone through the writing with a fine tooth-comb check your word count. You should now be able to finish the story within the limit.
When you have finished go through it again, smoothing out the flow of words, sanding down all rough edges. Change a word here, adding there, removing fluff so the eye flows along seamlessly. Keep an eye on word count, if it drops below the limit don't just fill it up again. Look at where the story needs it, if it does, then add it in.
Keep pruning until you have it perfected. Once it is complete, publish on your site, blog or forum and invite critiques. Good luck and enjoy your new 'improved' writing skill.