The answer to this question is really in the fire itself. Fire is a chemical reaction that occurs very quickly and gives off heat and light. The most common is the chemical action between oxygen and fuel. If heat and light are given off, you have fire! To make a fire, three things are necessary.
The first is a fuel, the second is oxygen and the third thing is heat. Paper or wood that is simply exposed to air does not catch fire: When the fuel becomes hot enough, oxygen can begin to combine freely with it and then it will burst into flames. Every fuel has its own particular temperature at which it begins to burn. This temperature is called the kindling temperature or flash point of fuel. When something catches fire, it is very important to bring the flames under control as soon as possible. This is especially true when a building catches fire.