If you've ever been to a restaurant, you may have seen clear bags filled with water hanging on the doors or tied together in the outdoor dining area. You may wonder, "What's going on here? Some crazy new way to keep the temperature just right? A way to save money on pitchers of water?"
The only reason these hanging bags affect the temperature is by accident. Their main purpose is to keep bugs away. To keep flies away, people hang these bags outside their homes, businesses, and even in their barns.
There are different ways to do the water-bag practice. Some supporters say the bag must have floating flakes of tin foil, while others say it should have a penny or two. Some clever websites even make money off of the idea by selling specially made water bags that can be used as repellents.
After all, flies spend a lot of time buzzing around places that are full of germs, like trash cans, dead animals, and animal poop. Then, full of germs, they swarm around your chicken sandwich. It's only natural that you'd want to keep them away. After all, flies are not only annoying, but they also spread disease.
But what good does a bag of water do? Does it even work? Experts and amateurs are split on the question. Let's look at the issue from both sides.
Why flies and water bags can't live together
Many people like the water bag method of keeping flies away, from restaurant owners to people who grill in their backyards. The internet is full of success stories that range from simple to amazing.
How does this method get rid of flies? Some people say that the flies think the clear liquid looks like the surface of a body of water. Some people say that the insect flies away when it sees its own reflection blown up. But the most common reason given by entomologists and people who file patents is that light bends in different ways.
When something clear or opaque, like a piece of glass or a bag of water, changes the path and speed of light, this is called refraction. Light rays, which usually move in straight lines, now curve. This effect is the cause of some optical illusions, like mirages, that can sometimes confuse people. Read How Light Works to find out more about how refraction works.
In theory, some insects, like the housefly, can find refraction just as confusing. It has a large number of very sensitive eyes that let it see in many directions at once.
Most of the insect's head is made up of two large, complex eyes. Each of these eyes is made up of 3,000 to 6,000 smaller eyes. These eyes can't move or focus like human eyes, but they give the fly a mosaic view of the world around it. Each simple eye fills in a small piece of the puzzle, just like a pixel fills in a small part of the picture on a screen.
A housefly can tell which way is north by looking at where the sun comes from. Some entomologists think that when an insect's complicated, sensitive eyes see light that has been bent, it gets confused and flies away.
Some people who like water bags say they keep all kinds of flying insects away, but most say they only work on insects with many eyes, like houseflies.
We couldn't find out how much water the bags need to hold, how many bags are needed, or where the bags should be hung for them to work best.
Still not sure? You're not alone. Let's hear some of the reasons why people aren't sure about this fly repellent.
A lot of people don't believe that water bags can keep flies away. One group, the MythBusters, said that this one was busted. Other critics often put this theory in the same category as modern superstitions and old wives' tales. They say that success stories happen because people mix up correlation and cause.
Imagine that a traveling salesman makes you an offer you can't refuse: He will give you a belt buckle that can stop shark attacks for only $19.95. You wear it for a week, and sure enough, no sharks bite you. Does this mean that the magic buckle on the belt works? Is there a real link between wearing the buckle and staying away from sharks? Does one lead to the other? To figure out how to measure this, you'd have to think about how often sharks attacked you before you wore the buckle and what other things could be making sharks stay away.
The placebo effect can make it seem like hanging water bags to keep flies away work even though they don't. In medical terms, this is when someone who thinks they are being treated for a condition feels better even though the treatment does nothing at all. People who think they are getting rid of pests could have the same effect.
But what if things keep getting worse? What if the placebo makes the problem I'm trying to fix worse? When Mike Stringham, an entomology professor at North Carolina State University, looked into how clear plastic water bags could be used to keep flies away, he ran into just this kind of thing.
Stringham did a 13-week field test by putting commercial fly repellents that work with water on two egg farms. Stringham figured out how active the flies were by looking at the spots where they left food they had just eaten. He came to the conclusion that houseflies were more active in places with water bags.
However, the study wasn't done in a place with natural light. The goal was to find out if the water bags could be used to reduce the number of flies on egg farms. The study didn't look into whether the water bags would work better if they were in direct sunlight.
So, do bags of water reduce the number of houseflies around homes and restaurants? There are good reasons to say yes, and there are also strong reasons to say no. Even so, water bags are still hung up near restaurant patios and back porches all over the world.