Is It Done Yet?

The hardest part of being a writer, for me, is knowing when something is truly finished. I read a short story that I think I completed days ago, and I find all sorts of problems to fix. I make the changes, and put it aside for a few days. Then I look at it again, and find still more that I want to change--but this time they're smaller things. Then the process starts all over again, until the changes become tiny alterations: a comma here, a different word there. I might go through this more than a dozen times, until I find nothing I want to change. Then I send it out and, usually, it comes back with a rejection note. So I read it over again and--yes--find stuff I want to change. Even in stories that I've published, I find passages that I would write differently if I had it to do over again. How do you know when it's done? I wish it was like cooking a turkey, and you could just stick a thermometer in it and yell "come and get it!"