Fur is the thick, soft coat of hair that covers many mammals. Unlike the single hairs on your head, fur grows in dense layers that trap air close to the animal's skin, keeping it warm in cold weather and protecting it from rain, snow, and scratches from branches or thorns.

Different animals have dramatically different types of fur. A polar bear's fur is thick and water-repellent, perfect for Arctic survival. A rabbit's fur is soft and fine. A porcupine has fur mixed with sharp quills for defense. Sea otters have very dense fur, with far more hairs per square inch than humans have on their heads.

Humans have used animal fur for warmth since prehistoric times, making coats, hats, and blankets. Today, most winter coats use synthetic materials that mimic fur's insulating properties without using animal pelts.

The word can also describe something that looks or feels like fur. A furry caterpillar has hair-like bristles covering its body. When mold grows on old bread, people sometimes say it's getting furry because the fuzzy growth resembles an animal's coat.