You’re in the middle of an old forest. There is a thin path through the trees, otherwise there are just trees, flowers and plant life in general. You are standing off to the side of the path in a patch of wildflowers. As you wonder where you are and how you ended up where ever it is that you are… you hear the galloping beat of a horse. It’s getting louder, coming towards you.
While you wait… imagine the story of the galloping horse. Where is it galloping from? Why is it in such a hurry? Is it running to something or away from something? Is there a rider or is the horse alone? Is this modern times or are you back in the past? As the hoof beats get louder, you have a steadily shrinking amount of time to come up with a story before the horse comes into view and your chance to create the story yourself will be gone.
When you can hear the horse in your head, start writing. But, be quick! Horses can run pretty fast.