Fire can spread quickly because most things around us release a lot of energy when they react with oxygen.

Combustion, or burning, releases a lot of energy in the form of heat. When there is a lot of heat concentrated in one area, a lot of unexpected things can happen. Since most things around us are made from materials with carbon and hydrogen, they are generally able to combust at higher temperatures. Carbon will combine with oxygen to form carbon monoxide and carbon dioxide, and the hydrogen will combine with oxygen to form water. These reactions release even more energy, and the fire continues.

In fact, plastics are naturally combustible because they're similar to the fuel you put in a gas tank; they contain a lot of carbon and hydrogen. (They're even made from products that are usually the result of breaking down fuels!) Because of this, almost all plastics have fire retardants put in them, special chemicals to reduce their flammability. If it wasn't for fire retardants, house fires would be a far bigger problem than they are today. .