You'll probably have to spell out exactly what is off limits. If you have specific shelves, etc., then those are your shelves and your possessions belong on them. I don't care for the casual way people sometimes regard other people's possessions. If you don't get permission, it isn't borrowing, it's stealing. Of course, generosity is a wonderful trait and should be encouraged. But if you specifically state what is not to be touched and/or eaten, then that's the way it should be. If she continues w/this behavior, then you have your answer.