"Well I saw that person's table take the felt and reuse it a bunch of times, and I only used mine once."
"That group dumped their oil right in the middle, and we poured our sample all over the water cup."
"I only sucked up the oil with the pipette once. That group did it over and over. Can we do that?"
Fourth graders are quickly seeing that in order to truly find out which materials work best in an oil spill clean-up process, the investigation that they design must utilize all the materials in a controlled manner to ensure the tests are fair. We spent a class period ensuring that our tests are as fair as possible, including setting a timer for how long a material can be used, how we dump the oil in, and how the material itself is physically used. While they controlled some initial variables (like the amount of water and oil in the cup), fourth graders are seeing that there are many variables that can affect the outcomes of an experiment. This allows them to see that their procedures can be written that much more specifically to guarantee a fair test.
Way to go fourth grade!