The burning process generates intense heat and hot expanding gases. In the open air, the gases dissipate, as does much of the heat. However, enough of the heat is transferred to the liquid gasoline in front of the flame to vaporize it and create a flammable mixture. Another term used to describe how flames spread is deflagration. Deflagration is an oxidative decomposition (burning) in which the propagation speed is slower than the speed of sound (subsonic). Although the terms explosion and detonation are often used interchangeably, they are two distinct things. An explosion is a sudden, rapid, and often violent release of hot expanding gases. Not all explosions are caused by explosives. Heating water in a confined space can cause a steam explosion; this type of accident was not unusual in the age of steam engines. Once the container fails, the hot expanding gases drive outward and carry pieces of the container along with them (shrapnel).