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. Sesame street is producing interactive youtube videos to teach science. While this is pretty neat, technologically, it would seem to me that this is the type of experiment that kids should be doing for real, not by clicking on some buttons on youtube.

It also makes me wonder what we could do to ensure greater transfer from lessons like this to high school science classes, where students are often unable to find any reasonable justification for why an object will sink or float. Sure, they all can spout the magic word “density,” but they might as well be saying “little green men” when you ask them how they know that to be the case. I know this, because I once really tried to teach this topic, devoting months to the subject of sinking and floating, adapted from the wonderful Physics by Inquiry. I found kids would try to pass of an experiment where they’d drop a marble and a pencil into the water, and observe the pencil floating and then say “see, density explains why they sink or float.”

Often, I’d reply with something like “all this experiment tells me is that things imprinted with No. 2” will float. And thus would begin a week long journey to really devise some experiments to tell us that, and along the way, students had to unpack the ideas of mass, volume and ratio.

Finally, the unit would culminate in students writing 8-10 page papers explaining sinking and floating from first principles. These students were juniors and seniors, who at the same time were writing papers in english about Hamlet and The Dubliners. And though it may sound basic, I can say I’ve rarely seen students as engaged as when they were trying to work out the chain of reasoning to explain how density predicts floating, and how this leads to understanding how a submarine can control its buoyancy.

I’m still not sure if it was worth it back then to take 2 months to study sinking and floating, but I can say that my students learned to make scientific argument perhaps better than any other students I’ve taught before or since.

That’s awesome. I agree on the value of learning experimental design. In fact, the kids would design experiments that would tell them that volume affected sinking and floating (a balloon with the same mass but different volume would sink or float) and then that mass affected sinking and floating (a film can with different numbers of pennies inside would sink/float). Then they saw that neither thing on its own was enough to predict sinking floating (a bowling ball with a larger volume than a balloon still sinks). Then they went out to look for combinations of mass and volume, and saw that the ratio of mass to volume did seem to predict sinking floating behavior perfectly. Then I asked them if we define this new thing, $density = mass/volume$ is it possible to do a control of variables experiment where you keep 2 of the 3 constant and change the 3rd to see how it affects sinking and floating, and they saw that you can’t, since density is defined in terms of mass and volume, not even a billion dollar research budget could create that experiment. It was beautiful, but I would always cringe when I would hear students quickly describe what we are doing “We’re learning about balancing, or we’re learning about density” for three weeks.