Rapid combustion is another form of fire, in which large amounts of heat and light energy are released, which often results in a flame at the same time. At times, large volume of gas is released during combustion and the sudden release of the gas creates excessive pressure that produces a loud noise. Such combustion is known as an explosion. During an explosion, there will be a primary wave build up and released, followed by a secondary wave. These high-speed pressure waves often cause tremendous damage and destruction. Even building structure may be damaged during the process. An explosion is normally followed by a fire, e.g. 9/11 incident. Combustion needs not involve oxygen; A typical characteristic of combustion would be hydrogen burns in chlorine forms the Hydrogen Chloride that will result in liberating heat and light.