the metal on the roof of a car is exposed to air & if something is exposed to air, then that something is in contact with air ->  the metal on the roof of a car is in contact with air  the metal on the roof of a car is in contact with air & air contains oxygen and water vapor ->  the metal on the roof of a car is in contact with oxygen and water vapor  the metal on the roof of a car is in contact with oxygen and water vapor & metal in contact with water and oxygen will rust -> the metal on the roof of a car rusts