flys in the house

I saw an article a few weeks back saying that a baggy filled with clear water and a few pennies in it hung in a room will keep flies away. They are not sure why it works but speculate it's got something to do with the reflection caused by the pennies in the water. I laughed when I read about it and a week later I was looking at a house to buy and low and behold they had a bag with pennies hanging in the middle of the downstairs room.