Scientific American: When a word becomes so popular you begin hearing it everywhere, in all sorts of marginally related or even unrelated contexts, it means one of two things. Either the word has devolved into a meaningless cliché, or it has real conceptual heft. “Green’ (or, even worse, “going green’) falls squarely into the first category. But “sustainable,’ which at first conjures up a similarly vague sense of environmental virtue, actually belongs in the second. True, you hear it applied to everything from …