Marrow is the soft, spongy tissue inside your bones. If you've ever seen a dog happily gnawing on a bone, it's trying to get to the marrow at the center, which is rich in nutrients and fat.

In humans and animals, bone marrow has crucial jobs. Red bone marrow produces new blood cells constantly: millions every second, replacing old cells that wear out. Yellow bone marrow stores fat that your body can use for energy when needed. Without healthy marrow, your body couldn't make the blood cells that carry oxygen, fight infections, or help wounds heal.

The word also appears in the phrase to the marrow, meaning deeply or completely. When cold weather chills you to the marrow, it's penetrated so deeply you feel frozen to your core. When someone says a story touched them to the marrow, they mean it affected them profoundly, reaching their deepest feelings.

Marrow bones from cows or other animals sometimes appear in cooking, especially in soups and stews, where the marrow adds a rich flavor. In some cuisines, roasted marrow on toast is considered a delicacy.