Turning a faint glimmer into a full story with the 5 Ws

Has this happened to you? 

You overhear a tantalizing tidbit of  a conversation or you happen to glimpse an interesting interaction between two people (or one person and a pet or an inanimate object or whatever!) and your writer's mind seizes on it because it just might be something that could turn into SOMETHING — a piece of fiction, an essay, a poem, an idea for an article or a book.

But when you sit down to start working on it, your creativity engines stalls out and you go nowhere. ...