Keeps flies away!!
Fill a ziploc bag half full with water. Drop 4 pennies into the bag and zip shut. Place bag near your picnic table or whereever you have trouble with flies. This will keep the flies away.
The reason this works? Millions of molecules of water presents its own prism effect and given that flies have a lot of eyes, to them it's like a zillion of disco balls reflecting light, colors and movement in a dizzying manner. Flies do not like this light and movement and will fly away.
Try it, you'll like it. (My friends say it works!)