Can a Bag of Water Really Keep Flies Away?

This old-fashioned trick has been around for years, and many homeowners still use it during barbecue season. Simply fill a clear sandwich bag about three-quarters full of water, add a few small balls of crumpled aluminum foil, seal the bag, and hang it near a doorway, porch, patio, or picnic table where flies tend to gather. Some people even use a few shiny pennies instead of foil.

No single scientific study has proven this method is consistently effective, but researchers and pest experts have proposed a few possible explanations. The shiny foil reflects sunlight in multiple directions, creating flashes of light that may confuse flies. The water inside the bag can also act like a magnifying lens, bending and scattering light as it passes through. Because flies have compound eyes made up of thousands of tiny lenses, these constantly changing reflections may disrupt their vision and make the area less comfortable to fly through.

While experts don't all agree on how well the trick works, it's inexpensive, chemical-free, and easy to try. At worst, you've used a sandwich bag, some water, and a little foil. At best, you may enjoy a more peaceful afternoon outdoors.