In order to power tomorrow's cars, researchers are scrambling to exploit the hydrogen-absorbing properties of carbon. Nobody really knows why carbon nano materials are good at storing hydrogen. Michael Heben of America's National Renewable Energy Laboratory, a pioneer in the field, believes that it is something to do with the structure of the nanomaterials surfaces. Molecules of the gas seem to fit into pores in these surfaces, though exactly why they prefer some pores over others is unclear.