The two most popular types of fuel for fuel cells are hydrogen and methane (or natural gas). While hydrogen fuel cells emit nothing but water vapor, you don't find a lot of hydrogen infrastructure for getting that fuel to where it's needed. On the other hand, there is a lot of infrastructure for natural gas, but those fuel cells emit carbon dioxide as a byproduct. There is a way for natural gas fuel cells to be considered a low-carbon energy source, but they have to get their fuel from a specific source. What is that source, and why does that qualify as a low-carbon source of energy?