An unopened DietCoke floats in water; a Coke Classic does not. On being asked for an explanation, a grade school student suggested that the cause was that Coke Classic has more calories. The instructor (telling this story on National Public Radio's Morning Edition) explained to the students that "calories are not real things but rather a measure of something" and that the real cause is that DietCoke is sweetened with NutraSweet while Coke Classic uses corn syrup and corn syrup is more dense than NutraSweet. All of this is of course correct. One can, however, ask another question: what does more dense mean? The definition of "the ratio of mass to unit volume" pops up from memories of our own science classes; yet, is density a thing and therefore a different kind of thing from a calorie such that more calories cannot cause a soft drink can to sink but more density can? How is it that we understand the word "density"?