31.   With hot-air systems, Frezzo said, air is heated instead of water.

32.   With a forced-air system, the central heating unit heats air that moves through a duct system around the house.

33.   With a radiant-heating system, Gelber said, the system functions not by heating the air in a room, but by heating the objects in it.

34.   At night, the burners are lit to heat the air in the bottom chamber, the part most resembling a standard hot-air balloon.

35.   Lightning heats the air to very high temperatures and creates magnetic fields in which the magnetic lines form interconnected loops.