Sometimes I get a paragraph or two down in sessions like this. Sometimes it's just an odd line. Occasionally it's a piece of dialogue that I know I want to get into some story somewhere. But I write it down. The only magic wand a writer needs after all is a pen and paper. I don't know why it is but when I brainstorm I have to physically write. I can't do this directly to screen.
And following on from my flash fiction theme yesterday, one lines are often all I need to get started on a story. So if you have a scrap of an idea, jot it down. It may well become a bigger idea. If not, it may be enough in itself to help you create a flash fiction story.