Rusting is an electrochemical process in which different parts of the same iron surface behave as anode and cathode, forming a tiny galvanic cell. At the anodic region iron is oxidised, Fe → Fe^2+ + 2e^-, while at the cathodic region the electrons are consumed by the reduction of oxygen in the presence of water: O_2 + 4H^+ + 4e^- → 2H_2O. Because oxygen reduction requires dissolved oxygen, the cathode forms where oxygen concentration is highest, typically at the edges of a water droplet exposed to air. The region depleted of oxygen instead becomes anodic and corrodes, which is why pitting often occurs under the centre of a droplet. The metal centre and impurity-rich zones may influence local potentials but do not define the cathode by oxygen availability. This differential-aeration mechanism is the NCERT explanation of rusting. Understanding corrosion in this way ties directly into the wider study of redox reactions and electrochemistry, where the same reasoning recurs across many problems. Carefully relating the data to the governing principle ensures the reasoning remains valid even when the numbers or species in the question are changed. Plausibility check: corrosion damage concentrates in oxygen-poor anodic zones while oxygen-rich zones stay protected, consistent with the cathode being oxygen-rich.