The cold feet of penguins are really a hot story.
When penguins stand on ice in winter, one would expect them to have really cold feet and lose a lot of precious heat. And that would have fatal consequences, especially in winter when food is scarce anyway.
In order to keep the loss of energy to a minimum, nature has equipped penguins, ducks and many other birds that live in cold climates with a fascinating system.
These animals have a natural heat exchanger. This means that their feet are always cold. Before the blood of these birds reaches the feet, it is drastically cooled so that no heat is lost and the feet are always at ambient temperature. The artery which
pumps the blood from the warm body to the cold foot lies directly next to the vein which conducts the 
blood back to the body. When the cooled blood returns again, it is reheated by the warm artery as well as numerous other blood vessels. The cold feet have the added advantage that they will not fuse with the ice.
Humans use this mechanism in order to save energy. For example, heat exchangers built on the same principle take the residual heat from exhaust air or wastewater of a building and re-use this with heat pumps for heating.