Sugar fire

Place a piece of cube sugar on a tin lid and try to set it alight.

You will not succeed. However, if you dab a corner of the cube with a trace of cigarette ash and hold a burning match there, the sugar begins to burn with a blue flame until it is completely gone.

Cigarette ash and sugar cannot be separately ignited, but the ash initiates the combustion of the sugar. We call a substance, which brings about a chemical reaction, without itself being changed a catalyst.