Fire needs oxygen to burn. When a fire starts inside a building, the floors, ceilings, walls, doors, and windows can constrict the flow of air. Breaking in to fight the fire thus carries the risk of opening a new airway. If that happens, a smoldering fire can expand explosively, bursting into roaring flames as it sucks air in through the new passageway. This sudden inrush of air to fuel a burst of fire has a name: backdraft.