# Does a penny in a bag of water repel flies?

**URL:** <https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880>\
**Category:** Factual Questions\
**Created:** [September 26, 2003, 1:36am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880 "2003-09-26T01:36:39Z")\
**Posts on this page:** 20\
**Page:** 1

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**Author:** ![tgirsch](https://avatars.discourse-cdn.com/v4/letter/t/3da27b/32.png) [@tgirsch](https://boards.straightdope.com/u/tgirsch)\
**Post date:** [September 26, 2003, 1:36am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/1 "2003-09-26T01:36:39Z")

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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,

- TJG

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [September 26, 2003, 1:42am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/2 "2003-09-26T01:42:24Z")

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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.

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**Author:** ![tgirsch](https://avatars.discourse-cdn.com/v4/letter/t/3da27b/32.png) [@tgirsch](https://boards.straightdope.com/u/tgirsch)\
**Post date:** [September 26, 2003, 1:44am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/3 "2003-09-26T01:44:53Z")

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Q.E.D. Is this documented somewhere?

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**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\
**Post date:** [September 26, 2003, 1:46am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/4 "2003-09-26T01:46:58Z")

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What’s the purpose of bags of water hanging in restaurants?

[http://www.straightdope.com/mailbag/mwaterbags.html](http://www.straightdope.com/mailbag/mwaterbags.html)

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [September 26, 2003, 2:41am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/5 "2003-09-26T02:41:49Z")

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The penny has nothing to do with flies. [The copper in pennies acts as an algicide](http://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&safe=off&as_qdr=all&q=penny+water+algicide).

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**Author:** ![Zenster](https://avatars.discourse-cdn.com/v4/letter/z/cdc98d/32.png) [@Zenster](https://boards.straightdope.com/u/Zenster)\
**Post date:** [September 26, 2003, 4:27am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/6 "2003-09-26T04:27:27Z")

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Only if it is held in the center fielder’s mitt.

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**Author:** ![tgirsch](https://avatars.discourse-cdn.com/v4/letter/t/3da27b/32.png) [@tgirsch](https://boards.straightdope.com/u/tgirsch)\
**Post date:** [September 26, 2003, 12:59pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/7 "2003-09-26T12:59:54Z")

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Blake:

Thanks. All of my archive searches included the penny, which wasn’t mentioned in the SD article, so I missed it.

Thanks again.

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**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\
**Post date:** [September 26, 2003, 2:53pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/8 "2003-09-26T14:53:30Z")

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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?

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**Author:** ![troub](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troub/32/14802_2.png) [@troub](https://boards.straightdope.com/u/troub)\
**Post date:** [September 26, 2003, 3:08pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/9 "2003-09-26T15:08:39Z")

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> [@](#):
>
> \*Originally posted by Musicat \*  
> \*\*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.

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**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\
**Post date:** [September 26, 2003, 3:34pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/10 "2003-09-26T15:34:09Z")

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**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).

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [September 26, 2003, 3:51pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/11 "2003-09-26T15:51:04Z")

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> [@](#):
>
> \*Originally posted by Musicat \*  
> 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?

A chemist has _already_ told you that this is reasonable.

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**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\
**Post date:** [September 26, 2003, 5:19pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/12 "2003-09-26T17:19:31Z")

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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.

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**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\
**Post date:** [September 26, 2003, 6:31pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/13 "2003-09-26T18:31:46Z")

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To answer the OP, [HGTV](http://www.hgtv.com/hgtv/gl_diseases_pests_insects/article/0,1785,HGTV_3580_1381705,00.html) 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,

> [@](#):
>
> Perhaps the reflections in the water make a difference, somehow scaring or confusing the flies. Entomologists know that flies are phototropic, meaning they are attracted to light, but they don’t know why these water bags repel flies.
> 
> “I have no earthly idea,” an entomologist from Iowa State University replied when I asked him how well and how this works…

From [John Carlson, MD/PhD (parasitology), Tulane University:](http://www.madsci.org/posts/archives/feb2002/1012966124.En.r.html)

> [@](#):
>
> I have tried to locate scientific research into the use of water-filled bags as fly-repellants, but have been unsuccessful. I’ve found a lot of people who believe that it works, but no scientific evidence…

He suggests a way it could be tested, then:

> [@](#):
>
> If bags of water do scare flies away, then there could be many reasons for it. The most common guess that I have read is that the flies might mistake the bag of water for a wasp nest. This point of view is stated by an unnamed Alabama pest control technician. Wasps hunt other insects for food, and so it makes sense that a fly would want to avoid a wasp nest. I have not seen any evidence that flies really do know to avoid things that might be wasp nests, but it could be true. In the end, this is only guessing because no one has done the experiments to make sure that that is the case.

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.](http://www.paghat.com/bottles.html) Or maybe moles. Or flies. Or ghosts. Or ghost-dogs. Take your pick. [snopes link](http://www.snopes.com/critters/wild/lawn.htm)

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.

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**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\
**Post date:** [September 26, 2003, 6:48pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/14 "2003-09-26T18:48:30Z")

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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.

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**Author:** ![gcarroll](https://avatars.discourse-cdn.com/v4/letter/g/65b543/32.png) [@gcarroll](https://boards.straightdope.com/u/gcarroll)\
**Post date:** [September 26, 2003, 7:30pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/15 "2003-09-26T19:30:30Z")

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\>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?

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**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\
**Post date:** [September 26, 2003, 9:40pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/16 "2003-09-26T21:40:03Z")

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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:](http://www.usmint.gov/about_the_mint/fun_facts/index.cfm?flash=yes&action=fun_facts2)

> [@](#):
>
> The cent was bronze (95 percent copper, and five percent tin and zinc) from 1864 to 1962.
> 
> In 1962, the cent’s tin content, which was quite small, was removed. That made the metal composition of the cent 95 percent copper and 5 percent zinc.
> 
> The alloy remained 95 percent copper and 5 percent zinc until 1982, when the composition was changed to 97.5 percent zinc and 2.5 percent copper (copper-plated zinc). Cents of both compositions appeared in that year.

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.

> [@](#):
>
> If flies are repelled by moving images, why do they seem attracted to the TV set at night?

That gives me an idea.

**How To Drive Your Fly Crazy** : Get a spotlight, some disco music, and a large mirrored ball…

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [September 26, 2003, 9:48pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/17 "2003-09-26T21:48:43Z")

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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](http://edis.ifas.ufl.edu/BODY_FA008) 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.

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**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\
**Post date:** [September 26, 2003, 10:21pm UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/18 "2003-09-26T22:21:30Z")

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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.

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [September 27, 2003, 12:24am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/19 "2003-09-27T00:24:23Z")

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-No problem. I should’ve put the copper sulfate link in my first post.

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**Author:** ![Roches](https://avatars.discourse-cdn.com/v4/letter/r/bc8723/32.png) [@Roches](https://boards.straightdope.com/u/Roches)\
**Post date:** [September 27, 2003, 12:55am UTC](https://boards.straightdope.com/t/does-a-penny-in-a-bag-of-water-repel-flies/203880/20 "2003-09-27T00:55:21Z")

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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. =))

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