I've just learned about this "home remedy" and I can see by googling it that other people use it successfully:

prevent flies bag pennies - Google Search (http://www.google.com/search?client=gmail&rls=gm&q=prevent%20flies%20bag%20pennies)

Can any of you tell me why and how this could work?

Thanks!

Probably. My theory. It has nothing to do with water and not much to do with pennies.

What I can picture is when I was a kid there was a reflective spinning ball on the ceiling of a roller rink.

I think the bag of water mimics this with no cost. It difracts (bends) the light in random directions this disorienting the fly. An object inside such as a penny or foil just makes it more random and occaisonally more intense.

I'm almost ready to bet that the roller rink ball would work too.

The disco ball might work, but it doesn't diffract the light like a bag of water does. It works quite well without the penny. I have seen it keep a house clear of flies hanging on the porch over a barnyard.

A mirror ball: Mirror Balls - Buy online here! (http://www.eliminatorlightingdirect.com/Mirror_Balls_s/3.htm?gclid=COyY3fChgKECFd1n5Qod7ATEug)