How long could you stand on Antarctic ice before your bare feet froze solid? A minute, maybe two? If you're an emperor penguin, you can do it for two months, and in wind chills as low as -75 degrees Fahrenheit (-59.4 degrees Celsius). Those naked bird feet may look positively frigid, but their special circulation acts as a kind of antifreeze to keep them just warm enough that they don't freeze.
Penguin legs and feet have evolved to lose as little heat as possible. Penguin feet hold onto heat by restricting blood flow in really cold weather, keeping foot temperature just above freezing. Penguin legs work like a heat exchange system; blood vessels to and from the feet are very narrow and woven closely together, cooling the blood from the body on the way to the feet and heating the blood as it returns to the body. Feet get cool blood so there's less heat to lose, while the body stays toasty.
This special ability is part of how penguins keep their eggs warm until they hatch. Male emperor penguins incubate a single egg on top of their feet for two months in the dark of winter while females are out feeding at sea. They also cover the egg with a flap of warm belly skin called a brood pouch to keep it out of the elements.
Nurturing doesn't stop there for these dedicated dads. If females haven't returned with food by the time the chicks hatch, male emperors feed their babies for a few days on a kind of "milk" made from special cells inside their throats.
Humans have the ability to restrict blood flow to their extremities in cold weather too, although not as dramatically as penguins. Your hands get whiter during freezing weather because there's less blood in them – it's been redirected to the core of your body to make sure vital organs stay warm.