A fuel cell consists of two electrodes sandwiched around an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat. Layers of materials with distinct electrochemical properties are sandwiched together to form a single galvanic cell. At the heart lies a membrane that can only be crossed by charged molecules. Gas-permeable electrodes coated with a catalyst adhere to this membrane, adding a layer on either side. The electrodes are in turn connected to a device that can utilize electricity. Hydrogen gas flows into channels on one face of the cell and migrates through that electrode, while the same occurs with oxygen gas along the opposite electrode. Spurred by a catalyst, favorable chemistry causes the hydrogen to oxidize into hydrogen protons and give up its electrons to the neighboring electrode, which thereby becomes the anode. This buildup of negative charge then follows the path of least resistance via the external circuit to the other electrode. It is this flow of electrons through a circuit that creates electricity.