Polar bear fur
The fur of polar bears appears to be white and makes them blend in with their environment. But the hairs of a polar bear are in fact not white at all. The hairs are hollow and translucent. The skin of a polar bear is black.
This is an optimal adaptation to the cold. The sunlight can reach the skin through the hairs. The black skin absorbs the warmth very well. The air within the hollow hairs is also warmed by the sun. This protects the polar bear from the cold by the warm layer of air that surrounds its body. The reflection of the sun beams gives the polar bear its white appearance. Most polar bears are not as white as a sheet, but rather yellow white or greyish white, depending on the incidence of light and amount of dirt in the fur.