I sometimes find myself trying to summarize and simplify the problems with hydrogen, only to find my listener's patience wear thin with all the technicalities. As such, I'd like to find and focus on one single, nearly insurmountable problem with hydrogen. For the moment, I've been using its relative inefficiency; that is, that it takes a lot of energy to create cleanly, that there's just no way around it, and that in a world with limited resources, it's not a sustainable way to power cars. But I don't know if that's the strongest argument, or even if it is strictly true. Any ideas?