A fire starts with the ignition of a substance by a source that provides the necessary heat to the substance. This source can be a matchstick, an electrical spark, or the effect of a burning lens, such as a magnifying glass. The required temperature for ignition depends on the material. Paper burns at temperatures starting from 190°C, wood at 280°C, and cotton at 450°C. Therefore, a pair of jeans does not catch fire quickly, while a piece of paper catches fire very quickly. Once a substance starts burning, its intensity depends on the supply of oxygen. For this reason, windows and doors of a building should always be kept closed in case of a fire.
Paddle steamers are ships that are driven by a large paddle wheel present on the side or at the rear of the ship. The wheels get their energy from a steam engine, which was fired with coal in earlier times. The ship is pushed forward when the blades of the wheel dip in water and shovel it backwards. Such paddle steamers are seen mainly on River Mississippi, which is very shallow at places. A flat, box-shaped hull is ideal for the river steamers because the blades need to dip in water partially, making it ideal for the Mississippi.e after the other. 
In a steam turbine, hot water vapours flow from jets onto a rotor having several propeller-type shovel blades. There is a high pressure in front of the turbine, while a low pressure is maintained at the back. As a result, the steam shoots at high speed through the turbine transferring its energy to the rotor. The movement of the turbine is converted into electrical energy, through generators. Steam turbines are used mainly in coal-fired engines and nuclear power plants. The turbines of large power plants are up to 6o m long and weigh several hundred tons. Several rotors are connected in them one after the other.
In a pressure cooker, the food is cooked at a higher temperature than in normal vessels. Pressure cookers are closed and air-sealed vessels that do not permit air or liquids to escape below a preset pressure. Upon heating, water evaporates inside the cooker and the steam pressure starts building up. The water then starts boiling at this higher pressure. This lets the potatoes cook faster. Pressure must be released before opening the cooker by letting it cool down or by opening a drain valve carefully.
The secret of the candle lies in the wick. A matchstick ignites the wick. The heat of the flame melts the wax of the candle. The liquid wax rises up in the wick, and more it evaporates, more higher it rises. The vapour burns. Wax is made up of carbon and hydrogen. Both elements react with oxygen during burning, whereby water vapour, soot, and waste gases like carbon monoxide and carbon dioxide are produced. This also gives the colours to the candle flame - yellow light from carbon and the blue light from water vapours.