Heating means making a building comfortably warm relative to a colder outside temperature. Old, primitive methods of heating a building or a room within it include a heater, the fireplace, and the stove. In ancient Rome, a heating system called a hypocaust, warmed a building by passing hot gasses from a furnace through enclosed passages under the floors and behind the walls before releasing them outside. The principal modern systems that are used to heat a building are classified as warm air, hot water, steam, or electricity. In the warm-air system, air heated in a furnace, rises through warm-air ducts and enters the rooms through outlets, while cooler air in the rooms passes into return ducts that lead back to the furnace. The air circulates through the system by convection; i.e, the tendency of a fluid such as air to rise when warm and sink when cool.