Yesterday, I asked where the mass comes from in a tree.
The answer is actually really simple: Carbon. Photosynthesis is the act of converting CO2 from the air into O2 and a bit of H2O.
It turns out that if you ask new Harvard graduates this question, the vast majority of them answer some variant of "It comes from the soil". When people think of photosynthesis, they don't think about the carbon that's left behind. They'll usually be puzzled as they answer, because in their hearts, they realize that "the soil" doesn't actually work as an answer, but they can't quite put all the pieces together.
If you ask 7th graders the same question right after they've finished their photosynthesis unit, they end up coming up with a variant of the same answer. Or they say it comes from the water the plant absorbs. Soil and water have mass, and that seems to the determining factor in their answer.
You see, 7th graders don't seem to get the idea that air contains mass - it's just the stuff that's around them, it doesn't "weigh" anything. Since they have always been exposed to the weight of air, it doesn't occur to them that it has any real properties at all. If you show them a block of dry ice, and ask them what it is, they'll say "It's ice". If you ask them to weigh it, they get that part. It's not until you follow that up and ask "So what's happening to the ice?" and they realize that the fog it's generating is disappearing into the air that they'll start figuring out what's happening - that the mass of the dry ice is being absorbed into the air. The very smartest of the kids will then put the pieces together and realize that air DOES have mass.
Yesterday's "quiz" netted over 70 correct answers and about 15 incorrect answers, I've got to say that I'm pretty impressed. The reality is that since I only let the incorrect answers through, it biased the sample towards correctness (several people mentioned that they'd read the other comments and realized that their first thought was wrong).
Valorie had one more question: Could those of you who got the answer right, and who are under 30, and who were educated under the American education system post a comment below?