"Clean." "Green." What do those words mean? When President Obama talks about "clean energy," some people think of "clean coal" and low-carbon nuclear power, while others envision shiny solar panels and wind turbines. And when politicians tout "green jobs," they might just as easily be talking about employment at General Motors as at Greenpeace. "Clean" and "green" are wide open to interpretation and misappropriation; that's why they're so often mentioned in quotation marks.