Ok, so we’ve established the fact that I’m a scientist. Now that we’ve gotten that out of the way, I had a question for the kids. We had all watched the “Water for Life” videos with Jay-Z in Africa. We all knew that clean drinking water was important. I asked them, “So how do you think we might clean water?”
Now, my purpose was not to go into all of the higher order water treatment processes at this point. I just wanted to talk about the very first thing you might want to do to clean up water (or anything else) — you want to get the big chunks out. Lets say you have a mixture of stuff you want to keep (water) and stuff you want to get rid of (chunks in the water), and you want to separate them. One way is a simple filter. A simple filter separates things based on their size. For example, a window screen or a pasta strainer are both simple filters.
To illustrate this idea, we made a mixture of blue marbles (our water) and various buttons (our chunks). I asked the kids to construct a simple filter that would take out the buttons (chunks), but let the water (marbles) go through. They were split into groups of two, and each group was given a cup with the bottom cut out and some toothpicks. My only directions were to push the toothpicks through the side of the cup however you want so that the marbles come out the bottom and the buttons stay in the cup. I told them that after about 10 minutes, we would get back together and see which group had the best filter.
Many of the kids really seemed to enjoy the problem solving part of the lesson. They liked trying and testing their designs. Others were really stuck on what to do. They asked me how should we stick in the toothpicks? how many toothpicks should we use? what direction should the toothpicks go? I said, “I don’t know. I’ve never done this before. Try it and see what happens.” Letting the kids test their designs during development seemed to keep them interested in improving their designs. Some didn’t want to stop, in fact.
So, after 10 minutes or so, I went around and tested each group with a mixture from a big bowl. I was careful to fill the filters with about the same amount and shake them about the same amount of time to keep things fair. Some told me it wasn’t fair that the mix they had used to design was not the exact same mix that I used to test. I pointed out that you can never scoop out the same bucket of water twice, because it moves around. They still didn’t think that was fair. Some also told me that it wasn’t fair that some of the buttons were the same size as the marbles. I asked why that was a problem, and they said that they were the same size, so their filter didn’t work. I mentioned that this is one reason why a simple filter isn’t always sufficient for treatment, and that even when filters work, they often don’t work perfectly. I also pointed out that maybe more testing with more sample mixtures might be a useful thing to do.
The last thing we talked about was what makes the “best” filter. They disagreed for a bit, but generally settled on the one that let the most marbles through, but kept the most buttons out. Another thing I noticed was that some kids were really good at making the opposite of what we were officially trying to do. They made filters that kept the water in the cup, but let the buttons go through. We talked about how even though this was the opposite of the original idea, it still made the separation we were looking for, so it might be useful in a different way.
Overall, the kids seemed to really start to get the idea that simple filters work based on size, but that the kinds of objects that can go through a filter depend on the shape, orientation, and 3D structure of both the object and the filter itself. Also, they seemed to agree that a good filter lets through more good stuff and takes out more bad stuff in less time than a bad filter.