The 5 word sentence, where each of the 5 words is identical is:

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Buffalo buffalo buffalo Buffalo buffalo.

The meaning is:

Buffalo buffalo (i.e. buffalo that live in Buffalo) buffalo (i.e a verb meaning to intimidate or fight) Buffalo buffalo (i.e. buffalo that live in Buffalo).

This is also explained here and a longer more complicated version is here.  One day this knowledge might make you the winner of a pub quiz!

Whilst we’re on the subject of short pieces of writing, here are a couple of flash fiction competitions:

• Early Works Press are offering £75 first prize plus two runners-up prizes of £10 for a 100-word story. Entry is £3.50 and the closing date is 31/8/2012. Full details here.
• Multi-Story want 600 words and offer prizes of £300, £100 and £50 for an entry fee of £5 (or £8 for 2 stories).  It’s an open theme and the closing date is 29th February 2012. Full details are here.