One of the themes we have here is that great science can be learned by using materials you have at home, and how the quick, easy science fair projects can sometimes teach the best science. Here is an experiment that involves paper towels. There are a lot of things you can do, like test which one is more absorbent or which ones are stronger like they show on commercials. Here is one where you use the paper towels simply as one of the materials in the experiment.
Filtering water is a common topic in middle school, as part of an ecology unit, or in the study of pond water, or when straight out studying how water filters work. Here is an idea for a simple experiment.
It can be a pretty simple experiment, you want to find out which sort of filtering system can take the most impurities out of rain water or pond water.
All you need is some jars and then make cones from a double layer of paper towels. (may-be you already did a test to see which brand holds up the best) which will act as the filter. figure out how to fold the paper towels to place them into the jars and then put common filtering substances like gravel, sand, carbon, newspaper, cotton and even the salt mixture you use in your home system on top of the towel. If you want, you could so some combinations, cotton and carbon, sand and gravel. Then you just pour the pond water into the jars and the paper towel that has the most stuff on it, filtered the best.
This can easily be used as a demonstration project, but it should be after you do your experiment and collect the data, then you can pick a sampling to demonstrate to your class. The paper towl with the most funk on it, will be easy to show to the classmates.
For middle school students, it will require being able to explain what you are doing and why, you chose those items. This is good practice for middle school students and one of the reasons why demonstration projects work so well with this age. A quick, easy science fair project is a great way to practice.