Video narration
How can a rusty bicycle develop rust without any visible battery? On its damp surface, microscopic regions become an anode, where iron releases electrons, and a cathode, where reduction happens. Both are on the same metal. The water film is an electrolyte: it lets ions move and completes the pathway. Rust is not simply oxygen attacking iron; the wet metal has become an electrochemical cell. At the anode, iron loses electrons. They travel through the bicycle's metal to the cathode, where oxygen takes them up. The water film supports ionic movement, and those products become rust. That is why wet, oxygen-exposed iron corrodes faster than dry iron: moisture connects the reactions. Why, then, does the bicycle rust without a battery? Because its thin water film turns the iron into a tiny battery-iron loses electrons at the anode, while oxygen uses them at the cathode.