Some of my favorite questions begin with the same four words: "What would it take..." What follows those four words can be just about anything, as in: What would it take to ... make solar energy as cheap and efficient as fossil fuel-based electricity?... routinely build zero-waste factories?... recycle 90 percent of the waste in my city within five years?... make organic foods cost-competitive with conventional ones? ... make airplanes operate on electric power? ... create zero-energy low-income housing? And so on.