Steam is the gas which is given off as a result of boiling water. The normal boiling point of water is 100?C. Unconfined steam will expand to about 1325 times the size of the water from which it came. If it is confined, it will build up pressure which can be harnessed to do work. Incidentally, it is worth noting that pure steam is actually invisible. The vapour associated with steam which we normally see is really small droplets of water which occur as a result of condensation. The work which can be extracted from steam is achieved by allowing the natural expansion of the steam as it cools. If the steam is carried away from the source of heat which produced it, it will cool and expand. This expansion can be used to do work, like pushing a piston inside a cylinder. Steam pressure can also be used to do work as well as expansion. The steam collected by boiling water in a boiler can be contained in the space above the water level while its pressure is increased as more and more water is boiled. Eventually, the pressure reaches the safe working level of the boiler. Spring-loaded safety valves are provided to allow steam to escape if the pressure rises above the normal working level. If you see safety valves "blowing off" steam, you will notice that the steam is actually invisible for a short distance above the valve. Only when it has cooled and expanded will the familiar white plume become clearly visible. During blowing off, the nature of expansion can also be seen as the plume of steam widens the further away from the boiler it goes. In a steam locomotive, both steam pressure and expansion are used inside cylinders to do the work of moving the machine. Both can be varied by the driver to regulate the power used by the locomotive under the varying circumstances of train operation.