CoachA fire needs three things at the same time: a fuel, oxygen from the air, and heat to start it. These three things are drawn as the three sides of a triangle, the fire triangle. If one side is missing, there is no fire.
Examples
Example 5.5 (Lighting a match)
A match head is rubbed on the side of its box. The rubbing heats it enough to start it burning: the heat side of the triangle. The wood of the match is the fuel, and the air brings the oxygen.
Example 5.6 (Blowing on embers)
Glowing embers that seem almost dead burst into flame again when someone blows on them: the blowing brings more air, and so more oxygen, to the fuel that is still hot.