Combustion is a chemical reaction that gives off heat and light. In most cases, combustion involves the rapid combination of oxygen with a fuel to produce burning. The fuel may be a solid, liquid, or gas. Combustion occurs, for example, when oxygen in the air reacts with charcoal in a barbecue grill. When oxygen combines slowly with another substance, the reaction is usually called oxidation. The rusting of iron is an example of oxidation.