Where Do Your Ideas Come From?

People often ask where I get my ideas. Some ask with interest; a few ask with a sense of alarm, as if they can't get over the idea of a church-going choir director dreaming up headless corpses and subtle poisons.
Most writers I know could answer with a question of their own: How do I get them to stop? Every day at least one idea floats through my head that might make a decent book. The problem is choosing one that I like well enough to flesh out the details and finish. In other words, the idea is the easy part.
An idea for an Arthur-and-Guinevere sleuthing team struck my fancy a while back, but I couldn't get interested enough to actually write it. I have several other started mysteries that got shoved onto the back burner. An abandoned idea might be due to other demands on my time (like edits), a plot knot that won't dissolve, or simple boredom: I don't like the story enough to finish it. I call these files "somedays", as in some day I might go back to them. (James Michener didn't write IBERIA for years after he took all the notes and outlined it. I'm no Michener, but still.)
If you are a storyteller, stories grow in your head of their own volition. The work part is writing a story down, toiling over it until it is no longer just a good idea, but a story that others can understand and enjoy.
So my answer to "Where do your ideas come from?" is that my brain is like a tree full of story blossoms. I have to pick the ones that seem promising and then work on them until they become fruit.