Polar bears look pure white, but the surprise is what's underneath. Their skin is jet black - perfect for soaking up the warmth of the sun. And their fur isn't actually white at all: each hair is see-through and hollow, like a tiny straw. The fur looks white because it reflects light, the way snow does.
They are excellent swimmers. A polar bear can swim for several days in a row, paddling along the surface of the icy sea, looking for floating ice to climb onto. Their huge paws act like flippers, and a thick layer of fat under the skin keeps them warm in freezing water.
Cubs are born tiny - about the size of a guinea pig - in a snow den built by their mum. They stay in the warm den drinking milk for about three months. When they come out for the first time in spring, the world is brand new to them and they tumble around in the snow like puppies.
Each year in October, polar bears gather near Churchill, Manitoba, waiting for Hudson Bay to freeze over so they can walk out onto the ice and start fishing for seals. Tourists travel from around the world to see them - the bears outnumber the people some weeks.