Amazing Trick for Repelling Flies at your next outdoor picnic or party! I filled up three glasses with water and a few pennies (about 4-5), placed them around the food, and watched. We watched the flies come in to land, see the water, and turn around and ZOOM AWAY. Glass + Water + Pennies = crazy fly repelling abilities. It was unreal. I’d say it reduced the flies by about 90%. At one point we looked up and the flies were just sitting on the deck railing staring at all the food.