This is one I owe to the Argentinian writer Jorge Luis Borges, who said that the image of a slug crawling along the edge of a razor blade motivated him to write one of his stories. Nothing so strange or awful need animate our own work but images can often be, as they were for Borges, the originator of great stories.
So, think of an image, something that comes into your mind and holds your imagination. Ask yourself where it came from and why it has come to you. You do not need to write your story around the image. Rather, let it spark your imagination. This could be a 500 word assignment.