About 250,000 people a day pass through Stockholm Central Station in Sweden, going about their frenetic business and generating a great deal of waste heat that had, until recently, just been ventilated away. But now, heat exchangers in the station’s ventilation system convert the excess body heat into hot water that is pumped into a neighboring office building, lowering its energy costs by as much as 25%. Harnessing body heat works well in Sweden because of their low winter temperatures and high gas prices, making it worth spending a little bit of money on electricity to move heat from building to building, rather than spending much more on heating with gas. But as a dwindling fossil fuel supply affects economies worldwide, this and similar ideas might find wider application.