Israel Siegel’s “gravity-powered shoe air conditioner,” patented in 1994, fits bellows into the heel of each of a pair of shoes, so that the natural act of walking pumps a refrigerant through two networks of heat exchange coils, one operating as a heat-absorbing evaporator and the other as a heat-delivering condenser.

Depending on how these networks are arranged, the shoe can serve as a foot cooler or a foot warmer.