One of the technologies being touted as the next great thing for our cars is the hydrogen fuel cell. If you've heard anything about them, it's that there are no harmful emissions, the only by-product is pure water, straight from your tailpipe. Of course, that's only part of the story. While it is true that your exhaust will be clean, that's only because hydrogen in a cell is not a source of energy the way gasoline naturally is—it's a carrier, like a battery. The energy to be stored in the cell has to come from somewhere else. Right now, the sources are the same as they've always been, relying heavily on fossil fuels. The emissions are simply moved from your exhaust to a power plant.