The large red and white balls are oxygen atoms. The blue balls are hydrogen atoms. The arrow shows the fuel cell storing electricity during the daytime when the water molecule (left) lands its oxygen atom and surrenders two hydrogen atoms (blue balls at end of arrow). At night, the reaction would reverse. The two blue balls would pick up an oxygen atom and release the electricity they stored. The Stanford team discovered that the efficiency of the fuel cell depended on having empty spaces on the fuel cell surface where oxygen atoms could land.