Any substance or device that can produce a volume of rapidly expanding gas in an extremely brief period. Mechanical explosives, which depend on a physical reaction (e.g., overloading a container with compressed air until it bursts), are little used except in mining. Nuclear explosives (seenuclear weapon) use either nuclear fission or nuclear fusion. Chemical explosives are of two types: detonating (high) explosives (e.g., TNT, dynamite) have extremely rapid decomposition and development of high pressure; deflagrating (low) explosives (e.g., black powder, smokeless powder; seegunpowder) merely burn quickly and produce relatively low pressure. Primary detonating explosives are ignited by a flame, a spark, or an impact; secondary ones require a detonator and sometimes a booster. Modern high explosives use either mixtures of ammonium nitrate and fuel oil or ammonium nitrate-based water gels.