How Does One Come Up With An Idea For A Book?

I was reading an article about R.L. Stine, the author of the 'Goosebumps' series. The interviewer asked him how he went about writing his stories. He replied, "I come up with the title and then I write the book."

This makes perfect sense to me, as that's exactly what I do. I came up with the title for Penelope Parker before I even started writing. Might be backwards, but the book fits the title, not the other way around.