Elon Musk has discovered a new passion in life -- and it could be Tesla's best product yet (TSLA)

Tesla Tesla CEO Elon is the best car salesman in history. And as CEO of SpaceX, he's declared his intention to retire on Mars. But what are supercar-fast electric vehicles and low-Earth orbits in the face of ... roofing. Against all odds, Musk has become perhaps the biggest booster in the history of the roofing business, thanks to a new Tesla product, the Solar Roof, that officially went on sale Wednesday. It's the first post-Solar City-acquisition product that Tesla is selling, and it sounds as...