How and why do the boys make fire?

The boys build a fire after Ralph tells them that they need a signal fire in order to help them to get rescued. They build the fire on the top of the mountain where it will be most clearly visible to any passing ships or airplanes. The boys use wood from a pocket of jungle growing near to the summit. The fire is eventually lit by Ralph, using Piggy's spectacles as a buring glass to focus the heat of the sun.