Press a handful of steel wool firmly into a glass tumbler and moisten it. Invert the tumbler over a dish containing water. At first the air in the tumbler prevents the water entering, but soon the level of water in the dish becomes lower while that in the glass rises. After the steel wool is moistened, it begins to rust. The iron combines with the oxygen in the air, and we call this process combustion or oxidation. Since the air consists of about one fifth oxygen, the water rises in the tumbler until after some hours it fills one fifth of the space. However, an imperceptible amount of heat is set free in the process.