The British branch of the online automotive auctioneer eBay Motors is tackling one of the more curious potential problems of global warming: dirty cars. As England suffered through one of its hottest summers in more than a century-one in which draconian water-use restrictions went into place in drought-stricken areas-eBay decided to challenge a U.K.-based automotive think tank to find a way to keep cars squeaky-clean with less water.