Because fire extinguishers exhibit a positive externality, the demand curve does not reflect the value to society of the good. Therefore, the social-value curve lies above the demand curve, and the social-cost curve is equal to the supply curve. The market equilibrium level of output occurs where quantity demanded equals quantity supplied, or where the demand and supply curves intersect. However, the efficient level of output occurs where the social-value curve and the social-cost curve (which is supply in this case) intersect. Therefore, the socially optimal quantity is greater than the quantity that the private market would naturally reach on its own.