Burning is the reaction of oxygen with a fuel of some kind, often containing carbon or hydrogen. When a molecule of fuel reacts with a molecule or atom of oxygen, usually a certain amount of energy is released. This is because the oxygen bond to carbon or hydrogen is very strong. (This works both ways: you have to add energy to break an oxygen bond. Allowing an oxygen bond to form, releases energy.) Atoms are made from protons, neutrons, and electrons, and sometimes the energy released from the oxygen bond goes to excite an electron into a higher energy state. When the electron falls back to the lower energy state, it releases the energy as light.

The chemical reaction known as fire heats up the material being burned. When material is hot, it glows, whether it is solid (glowing coals) or gaseous (flames).