Every human being, no matter what he is doing, gives off body heat. The usual prob-lem is how to dispose of it. But the designers of the Johnstown campus of the University of Pittsburgh set themselves the opposite problem-how to collect body heat. They have de-signed a collection system which utilizes not only body heat, but also the heat given off by such objects as light bulbs and refrigerators as well. The system works so well that no con-ventional fuel is needed to make the campus's six buildings comfortable. Some parts of most modern buildings-theaters and offices as well as classrooms-aremore than amply heated by people and lights and sometimes must be air-conditioned even in winter. The technique of saving heat and redistributing it is called 'heat recovery. ' A few modern buildings recover heat, but the University's system is the first to recover heat from some buildings and re-use it in others. Along the way, Pitt has learned a great deal about some of its heat producers. The harder a student studies, the more heat his body gives off. Male students emit more heat than female students do, and the larger a student is, the more heat he produces. It is tempting to conclude that the hottest prospect for the Johnstown campus would be a hard-working, overweight male genius. Until recently, body heat has caused problems because it ______.