原文来自Lifehacker, 很早就看到了，只是一直没时间翻，不能再偷懒了，不然人又要懒散了。

Repel Flies with a Bag of Water

By Rosa Golijan,

9:30 AM on Mon Jul 6 2009,

104,190 views

We’ve featured many, many, many different ways to deal with mosquitoes using DIY solutions. But what about the even more common household fly? Turns out all you need to deal with that is a plastic bag or two.

Photo by brookage.

RickyC wrote in to tell us that he recently visited a lakeside restaurant and noticed some clear plastic bags filled with water hanging from the railings. When he inquired about the bags, a waiter explained that they were used to repel flies.

Question-and-answer web site The Straight Dope gives more detail on why this trick works:

Evidently, houseflies, being highly edible and defenseless, are nervous types, and don’t like to sit still when they see something moving nearby, because it could be a predator. The water bag acts a bit like a lens—try it some time—in which the movements of people in the area are reflected. Even if the fly is too far from the action to see it directly, it can see a shifting of light and dark in the water bag, which it interprets as nearby movement, and it will fly away from the bag.

So, next time you have a fly problem, grab a few plastic bags, fill ‘em up, and let us know how this trick works for you. Likewise, share how you keep unwanted insects out of your backyard—including whether or not you’ve tried this method before—in the comments.

What’s the Purpose of bags of water hanging in restaurants? [The Straight Dope]