Millions of water molecules in water cause a special prism effect, which means that so many water molecules refract light in a very special way. When you add coins, the cumulative effect of refracted light will also increase.
Flies have a very unique way of looking at things. Their eyes are made up of thousands of identical lenses, that is, they see thousands of reflections on the same image.
For flies, this simple water bag looks like many disco balls that reflect light and move shadows, making them dizzy. In this case, they will eventually become uneasy and feel vulnerable to predators. Flies are food for many other animals, so flies feel fragile and won't wander around. They think it's too dangerous.
This method is not only effective, but also low-cost, harmless to people and will not pollute our food, air and water like chemicals. Very funny, sometimes the best solution is the simplest!