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We often see the negative side of explosives on the news--images of twisted metal on damaged buildings or debris and rubble in a war zone. But explosives also help us construct tunnels, mine underground, remove obstacles during road construction, extinguish oil well fires, inflate automobile air bags, and destroy hazardous wastes. An explosive is a stable material that, upon stimulation, very rapidly changes from a solid or liquid into a hot, expanding gas. The sudden release of energy and the accompanying pressure exerted on the surrounding materials by the expanding gas constitute an explosion. If you confined an explosive, you would increase the likelihood of the detonation by increasing the speed of the reaction. For example, when gunpowder is confined within the paper wrapping of a firecracker, it explodes when ignited. However, the same powder sprinkled in the open simply burns when ignited. In the confined state, the heat generated at the point of ignition will immediately ignite a large amount of surrounding material. The expanding gases from all the "burning" gun powder will act together, resulting in an explosion. Explosives fall into two categories: detonating and propellant. Detonating explosives are further classified as initiating (primary) or secondary Initiating explosives, the more sensitive of the two, must be handled with extreme care. They require only a low energy stimulus, such as being touched with a hot wire or tapped with a hammer, to explode. For that reason, they are put into blasting caps. Secondary explosives are less sensitive and can burn without producing an explosion. They only detonate by means of a severe shock, delivered by another explosive (usually a blasting cap) placed in or near them. Since they are relatively stable, large amounts can be moved and handled safely. The velocity at which explosives detonate determines their function. Explosives of low detonating velocity supply a slow push or heave. Explosives with a high detonating velocity have a bursting or shattering effect. Propellant explosives are used for firearms, rockets, general engineering, and demolition work. They differ from detonating explosions in that an avenue of escape is provided for the expanding gases. For example, when the explosive propellant in the space shuttle solid rocket boosters is ignited, all the gases from the resulting continuous explosion escape out the bottom, providing upward thrust. Unlike a detonating explosive, not all the explosive material is burned at once in a propellant explosive. Industries rely on small explosive charges for a number of purposes. In metallurgy, for example, metals can be pressed into dies, extruded, or welded together by means of such explosions. New metal alloys have been created that way.