If flames are visible from the outside when firefighters arrive, they immediately go into an aggressive attack strategy trying to ascertain if they can still safely save lives. Next they direct water to extinguish the blaze at the heart of the fire. Water simultaneously cools the burning debris (lower temps mean fewer flammable gases being generated) and can limit oxygen's ability to fuel the fire. Firefighters, on average, use nearly 3,000 gallons of water on a house fire. Firefighters may also vent off hot smoke and gases either by breaking open upstairs windows or cutting a hole in the roof. They may also use dry chemicals to retard fire spread and extinguish flames.