The answers range from the serious to the silly, but one that I really like is:
"Everybody walks past a thousand story ideas every day. The good writers are the ones who see five or six of them. Most people don't see any." —Orson Scott Card
Your assignment as you go about your regular life today is to notice one thing—just one—that would make a good story.
(Okay, you can notice more than one, but notice at least one.)