What is fire, and why is it hot?

Fire is the burning products of vapours of a fuel. Whenever something burns it is first made into a vapour and mixed with oxygen, so when you have a fuel, such as paraffin or diesel, it works much better if it goes up a wick. This is because the heat can make vapour come from the wick to mix with oxygen in the air to cause a chemical reaction between the hydrocarbons in the fuel and the oxygen. When this happens, you get complete combustion. The flame that you see if just the hot gasses and soot particles, which are so hot that they glow. Gas burns blue because it burns clean, and so there are very few soot particles in the flame to glow yellow.