What's the point of the pennies in the bag of water to keep flys away?

Here you go:

https://www.hunker.com/13405986/how-to-repel-bugs-with-pennies-water-plastic-bags

My mom used to swear by this! https://animals.howstuffworks.com/insects/water-bags-repel-flies.htm

If you will remember your biology from days in school, a housefly has large complex eyes made up of thousands of simple eyes that don’t move or focus. The fly bases his movement by light and the refracted light coming through the water in the plastic bag confuses the fly causing him to move on to a place that is easier on the eyes. And when you have a few thousand to deal with at one time, the easier, the better. At least this is the explanation that some entomologists are giving to this new insect remover now spreading across the Internet and making its way to backyard gatherings. However, others are calling it old wives’ tales and modern superstition.

I have tried it several times and it doesn't work....

As far as I know, the pennies are not needed. A bag or jar of water will keep flies away because as they approach it they see an enlarged image of themselves and fly away.

I believe it has to do with the reflection of the bright light that will come off of the pennies and into the eyes of the flies that will keep them away.

https://www.dailymail.co.uk/femail/article-6694043/Why-putting-coins-inside-zip-lock-bag-filled-water-ultimate-hack.html

I found this, but it says nickels work better: https://www.youtube.com/watch?v=dLEWz25PvFA

Hi! It helps weigh the bag down since it's the water molecules that trick the eyesight of the multiple eyes a fly has, if I understand the theory.

Apparently so.... Ill have to try

https://holistichorse.com/health-care/keep-flies-away-with-water-and-pennies-2/