An explosive is a substance that will burn up or decompose almost instantly, producing intense heat and a large volume of expanding gas. This rapid change of state is called an explosion. Depending on its nature, an explosive may be detonated by apply heat, by shaking, or by striking. The sharp sound, or report, of an explosion is due to the air waves set in motion by the explosion's force. Gunpowder is considered a low explosive, but is not discussed in this section.