This is a dumb question but I can't find a definite answer. If I have, say, a nylon groserry bag and I weigh it *when is open*, then I close it, then weigh again. It's quite clear that, depending on how I close it, different amounts of air will be locked in the bag so it will weigh differently, but, when it is open, can I account for *any* of the air that is "inside" the bag? In a conversation I had, I argued that when the bag is open, none of the air "inside" it counts, at all, so when you close it, it always, neccesarily, weighs more. How is it?