Flash fiction is a comparatively recent author challenge.
There is a strict word count and the piece, not counting a title, should be an exact number of words. Exact! In these cases the number of words is 100 (could be 200 or 250 - whatever is selected), except for the last which is 210. Do the authors hit the target spot on? The last 100-worder, by the way, was written by a 14 year old.