I am gonna tell it to you now since, truth is, I probably won't remember to tell you next summer when the weather is warm. And, the other reason is that our guests have asked us about this so many times that we were thinking about writing a little blurb in the welcome folder that they see when they arrive. Doing that would keep us from explaining it over and over.
We discovered this on a jaunt somewhere,and, like many things, once you have seen it, then you notice it everywhere after that. A clear water-filled plastic bag was hanging up over the door. Yep, just water in perhaps a quart-size clear plastic bag that was tied with a string or wire and hung near the outside doorway. So we asked about it. The reply went something like this, "Well, it keeps flies from going inside the house. Don't ask me how it works; I don't know. I am tired of telling people about it."
So we tried it ourselves at our house, at the cottage and the cabins. We got the same response from many people; "what the heck it that?" Others told us, "We have known about that since we were kids." One of our friends who lives in Ireland said that her parents had always filled bags with water to keep out flies.
What do we think after trying it for a summer? We think it works. We had very few flies inside this year. How does it work then? We don't know, but the most consistent explanation offered when we asked was something about the light refraction in the clear plastic bag somehow confuses the multi-faceted eye of the flies. After that, your guess is as good as ours.
We even checked it our on Snopes.com. They don't say it is true or false. They say "Undetermined."
So, try it for yourselves next summer (or even now, as there are some flies still hankering to be indoors). And remember to tell us how it works. You have nothing to lose but your flies. The only downside is having to repeatedly answer the question, "What the heck is that thing?"