Keeping Flies Out

An e-mail came to ESP recently claiming to have an answer to a common problem. The problem was how to keep flies out of the house. The solution was to hang up a bag of water near where the flies might try to come in. Does this really work? It sounds like a good chance to do a scientific investigation.


The Elementary Science Program has a good outline for experiments. It includes: question, materials needed, step by step procedure, variables to control, what happened (data), conclusion (what the results mean), and new questions.

So the question would be, “Does hanging up a bag of water keep flies from coming indoors?” 

The first step was to find out more about how the bag of water was supposed to be set up. A Web search gave several descriptions of what to do. They said to use a clear plastic bag, about half full of water. Some said to put several pennies in the water. Some did not say to put anything in the bag except the water. Some said to hang the bag above the open door or window, some did not.

The materials would include a clear bag to put water in, some water, and maybe some pennies. Oh, don't forget some tape or a thumbtack to hang up the bag of water.

A procedure would be to have two rooms where a window could be left open. Hang a plastic bag partially full of water outside of one open window. Don’t hang a bag over the open window of the other room. And then wait to see if there is a difference in how many flies come in. It might be good to keep both rooms closed off from the rest of the house so any flies that come in won’t go to the other room in the experiment.

It is important to keep everything the same between the two trials except having a bag of water near one window. That's what we mean by saying you need to control variables. You need to keep everything the same except what you are testing. Can you think of things that should stay the same? The windows should be open for the same length of time. Oh, and the temperature and time of day should be about the same. Make a list of any other variables you can think of. Can you think of ways to keep things the same in both parts of the experiment?

Now try it and write down what happened. How many flies came into each room? Did you notice anything else, like flies that stopped and did not come in?

Next, write down what it means. Does the plastic bag of water really seem to keep flies out?

Scientists usually come up with more questions after doing an experiment. Maybe you wonder if where you put the bag matters. Does putting pennies inside the bag matter? Does the size of the bag matter? Does it help keep out bees or wasps?

Well there you go, you just did a real science experiment. Was it fun? Did you learn anything?

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