All around Memphis, I have seen places, mostly restaurants, hang a baggie filled with water and a single penny in the bottom, typically near doors and windows. When I asked about this peculiar arrangement, I was told that this was done to repel flies.
First question is, does this actually work?
Second question is, if so, how? 
Thanks in advance,
Yes, it works, however the penny is unnecessary. It’s the light reflecting off and refracting through the bag of water creating the illusion of motion that drives off flies.
Q.E.D. Is this documented somewhere?
What’s the purpose of bags of water hanging in restaurants?
Only if it is held in the center fielder’s mitt.
Blake:
Thanks. All of my archive searches included the penny, which wasn’t mentioned in the SD article, so I missed it.
Thanks again.
Interesting theories being presented here without a flyspeck of proof. No, a SD Staff Report by Doug with much speculation and absolutely no references does not constitute a high level of proof in my book.
Claim #1: Houseflies are repelled by small bags of water, possibly by images refracted or reflected by the crude lens formed.
Claim #2: One copper penny in a small baggie of water is sufficient to prevent algae growth.
It would seem that Claim #1 would not be that hard to test. Get some houseflies, put them in a large container along with water-filled baggies, milk-filled baggies (to allow reflection, but prevent the refraction), some opaque baggies, and maybe a water-filled baggie behind a tiny TV, to provide the moving images. Then count the number of flies on or within a predetermined distance of each object. Repeat until conclusive. Then use other insects, because the claim has also been made that only houseflies are so affected. Are moths and gnats immune?
But until a test is done, Q.E.D., I will be quite doubtful of your explanation. Any cites handy?
As far as Claim #2, perhaps a chemist could tell us if this is reasonable. Does copper have anti-fungal properties, and if so, how much such action can we expect from immersing a penny in a quart of water?
Not fungus, algae. Anyway, shouldn’t this be easy enough to test? Two identical bags of water, in the same environment, one with a penny and one without? I think a quart of water is a liberal guess, anyway. . .I was thinking something like a small sandwich baggy–I’ve never seen a bag of water hanging anywhere, but I wouldn’t think it’d be very big. A concern I would have about hanging an old stale bag of water around the house would be mosquito breeding.
troub, I assume we are talking about Ziplok-type sealed baggies, so it seems unlikely that mosquito larvae could get in or out of one (or at least you could see it growing and take action!).
My ignorance is showing when I equated algae with fungi; neither chemistry nor biology is a strong point.
While the “flies are repelled” claim would have to be tested by experiments, I suspect the penny/algicide claim could be calculated on paper. We have X quantity of solid copper, X% pure, immersed in X quantity of H[sub]2[/sub]O, at room temperature and near-sealevel pressure. How much of the active copper ingredient will be produced/dissolved, and how much affect would this have on a known quantity of algae?
Your test of two baggies, one with & one without, might not be conclusive. What if neither developed algae over time? You might have to innoculate some at random with known algae first, and some of those with pennies, some without, to take care of the possibility that algae dies normally, without pennies. And more than two would be better to avoid chance happenings in your bags.
It occurs to me that if the “flies are confused by moving images” hypothesis is correct, a fresnel lens like the ones used on the back windows of vans would provide a similar effect, and be a little more cosmetically appealing, yes? How about a mirror? So a fresnel lens and a mirror would be on my list of items to test in the fly-box proposal above.
Another way would be to use normal flies and blinded flies (I’ll stock up on micro-blindfolds, teehee).
A chemist has already told you that this is reasonable.
No offense, Squink, if the chemist is indeed you. But the link you posted was only a Google search list for “penny+water+algicide”. In that list of 37 hits, many refs to “penny” were “Mr. Penny,” a person, not a coin, or text like “save a penny…”, “it’s worth every penny”. Only one link actually mentioned algicide as part of a gardening discussion, and I didn’t see any way of calculating the amount necessary for the desired action. “…a copper penny in water is toxic to fish life…” doesn’t necessarily translate to toxicity to other life. I did not see a link that said “one small copper disk in a quart of water will surely prevent algae growth.” If I missed the direct link and you can supply it, I will stand corrected.
I will accept, especially if your background/education falls in the chemistry field, that some copper compounds will have adverse effects on some primitive life. Now what we need to establish is:[ul][li]Will the chemical reaction of a single penny in some quantity of water produce a compound toxic to commonly seen strains of algae, and in what quantities over what time, andIs this quantity & time adequate to produce the claimed effects?[/ul][/li]And, although you may think I am being too pedantic, a toxic copper penny action is a lot easier for me to accept than the claim that flies are effectively repelled by moving images.
To answer the OP, HGTV has this to say:
[quote]
You may get some questions as to why you have little bags of water taped to your door. Here are some explanations you can use: [ul][li]It attracts flies[]It scares flies[]If it freezes we know it’s cold outside[]If it’s boiling we know it’s hot outside[]to see if anyone is paying attentionI don’t know, ask the Bugman. This is his idea[/ul][/li][/quote]
As to why/if it works,
From John Carlson, MD/PhD (parasitology), Tulane University:
He suggests a way it could be tested, then:
No mention of phototropic phenomena.
I admit this may not be closely related, but it smacks of the same kind of thinking – some people believe that water bottles left in a yard will repel dogs. Or maybe moles. Or flies. Or ghosts. Or ghost-dogs. Take your pick. snopes link
No, I have not been skipping over good references that support the theory in favor of skeptical ones; these are all I have been able find on either side of the question. So far, I would say the “flies are repelled by waterbags” postulate is looking pretty unlikely and falls squarely in the Urban Legend category.
But that’s just my opinion.
I’m willing to entertain the idea that something might work as long as we have either 1) evidence that it does work, or 2) a plausible mechanism by which it might work. In this case, the mechanism isn’t plausible (why would a fly be scared of a little bag of water meters away, but land carelessly right beside my hand?), and we haven’t found any measurements saying that it does work. So I’ll file this one in the “disbelieve” column, at least until we have something that would make me move it to either the “undecided” or “believe” columns of my brain.
>A chemist has already told you that this is reasonable.<
Has anyone considered that US pennies are not actually made of copper anymore?
If flies are repelled by moving images, why do they seem attracted to the TV set at night?
Good point, gcarroll. But U.S. pennies for 150 years have been either mostly copper, or copper-clad except for some in 1943. From the U.S. Mint:
So if the amount of copper is important, either an older penny (pre-1982) should be used. But a newer copper-plated zinc one, since the copper is on the outside, would present the same surface area of copper to react with water & dissolved minerals at least until the plating wore off.
We need to know how fast this reaction is, and if it would be sufficiently toxic to algae in that very small quantity.
But as far as flies are concerned, it’s beginning to look like Staffer Dave may need to revise his SD Report just a lil.
That gives me an idea.
How To Drive Your Fly Crazy: Get a spotlight, some disco music, and a large mirrored ball…
Pennies in the US are now made primarily of a zinc alloy. They still have a copper cladding. That’s what makes them penny colored.
Soluble copper salts have been used as algicides in lakes and streams for decades. This page gives a nice overview of its use. 1.5mg/L CuSO[sub]4[/sub] is effective at controlliing algae under fairly typical water conditions.
Keeping a 500 ml plastic water bag algae free would thus require only ~0.75 mg of soluble copper salts. A little grime on the penny, a little acid from the water, or acidic monomers from the baggy itself are all that’s needed to reach an algicidal Cu[sup]2+[/sup]concentration.
Thanks, Squink, just what we needed, Cu-wise. Sorry I ever doubted you! 
So it seems we have answered 1/2 of the OP, but perhaps not the important half.
-No problem. I should’ve put the copper sulfate link in my first post.
gcarroll/Musicat: I tried to post about that, but it didn’t work… =) (I was a bit concerned about copying from the US Mint site, since federal government sites have all kinds of usage restrictions…)
Squink, it seems that soluble Zn salts are also algicidal, but, as you mentioned, pennies are still clad with Cu, so all the Cu is on the outside. Thus, there’s about 63 mg of Cu on the surface of a penny (2.5 g, 2.5% Cu). Perhaps the best way to get it into solution would be by adding some vinegar – copper(II) acetate is very soluble, and you wouldn’t need much acid. Rainwater, and probably city water, probably contains enough carbonic acid (and possibly some sulfuric acid) to get sufficient Cu[sup]2+[/sup]. (I just learned how to do superscripts because of your post – thanks. =))