Hanging a plastic bag filled with water and a few pennies is a folk remedy claimed to repel flies from porches, patios, and outdoor dining areas. The idea has been popular for decades, especially across the southern United States and in countries with warm climates. Despite its widespread use, no controlled scientific study has confirmed that it works.
How It’s Supposed to Work
The most common explanation is that sunlight passing through the water bag creates refracted light patterns that confuse or frighten flies. The pennies, being shiny and copper-colored, are said to amplify this effect by adding points of reflected light inside the bag. Supporters of the method claim that the shifting light resembles the movement of a predator or creates visual chaos that drives flies away.
This explanation sounds plausible on the surface because fly vision is genuinely different from ours. Flies see the world through compound eyes made up of thousands of tiny units called ommatidia, each capturing light from a slightly different angle. Their visual system is built to detect changes in brightness and motion with remarkable speed, responding to shifts in a moving object’s trajectory in as little as 20 milliseconds. The theory suggests that refracted light from a water bag overwhelms this sensitive motion-detection system.
Why the Theory Falls Apart
Fly eyes are sensitive to motion, but they’re also highly adapted to filtering out irrelevant visual noise. A fly’s brain contains specialized neurons in a region called the lobula plate that respond selectively to motion in specific directions. These neurons help flies distinguish between background movement (like wind blowing through leaves) and meaningful movement (like an approaching hand or a potential food source). A stationary bag of water producing scattered light patterns is exactly the kind of stimulus a fly’s visual system is designed to ignore.
Flies also use what scientists call an optomotor response, which means they tend to follow and stabilize against broad movements in their visual field rather than flee from them. A small, fixed light source wouldn’t trigger the kind of alarm response the folk remedy assumes. If anything, a bright, reflective object near food could attract flies rather than repel them, since many fly species are drawn to light.
The core problem is simpler than the neuroscience, though: nobody has published a peer-reviewed study showing that water bags reduce fly activity in a controlled setting. University entomology departments and agricultural extension programs, which regularly evaluate home pest control methods, have not endorsed the practice. When the method has been informally tested, results have been mixed at best, with most observers reporting no meaningful difference in fly presence.
Why People Believe It Works
Confirmation bias plays a big role. If you hang a water bag and happen to have fewer flies one afternoon, you credit the bag. If flies show up anyway, you don’t count it as a failure. Fly activity naturally fluctuates throughout the day based on temperature, wind, humidity, and nearby food sources. A quiet afternoon on the porch feels like proof when you’re looking for it.
The method also benefits from social reinforcement. Restaurants and markets in fly-heavy regions display water bags prominently, which makes the practice feel tested and validated. Seeing it “everywhere” creates the impression that it must work, even though widespread use and actual effectiveness are two very different things.
What Actually Keeps Flies Away
Flies are driven almost entirely by smell. They seek out decomposing organic matter to feed and lay eggs, so the single most effective deterrent is removing what attracts them in the first place. Pick up pet waste regularly, clear fallen fruit from your yard, and keep garbage bins sealed and positioned as far from your door or patio as possible. Taking out organic waste at least once a week makes a significant difference in how many flies breed near your home.
For outdoor gatherings, fans are one of the most reliable physical deterrents. Flies are weak fliers, and even a moderate breeze makes it difficult for them to land. A box fan or oscillating fan pointed across a dining table works surprisingly well.
Commercial fly traps that use inverted cone designs with food-based attractants are effective outdoors, though they come with a tradeoff: the bait smells terrible. Place them well away from where people sit, and they’ll draw flies toward the trap and away from your food. Sticky fly ribbons work in smaller, semi-enclosed spaces but aren’t powerful enough for open outdoor areas or heavy infestations.
If flies are a persistent problem around your home, the issue is almost always a breeding source nearby. A forgotten compost pile, an uncovered trash can, or even a pile of grass clippings can produce thousands of flies in a matter of days. Finding and eliminating that source does more than any deterrent ever will.