Metro stations in Paris and Stockholm are toasty even in wintry temperatures because of the aggregate heat emanated by thousands of transit users. Enterprising and ingenious engineers and architects in both cities have figured out how to capture that heat and direct it to nearby buildings. The caveat: the buildings must be *very* nearby. If the body heat has to travel more than 100 or 200 yards, it dissipates before it can be used. This, a quirky but real benefit of high density development and transit-orientation.

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