Bone marrow is the soft, spongy tissue inside your bones where your body manufactures blood. While bones might seem solid all the way through, most of them contain this remarkable factory at their core, constantly producing millions of new blood cells every second.

Your body needs three main types of blood cells, and bone marrow makes them all: red blood cells that carry oxygen throughout your body, white blood cells that fight infections and disease, and platelets that help your blood clot when you get a cut. Without bone marrow doing this essential work, you couldn't survive.

When someone has certain serious illnesses like leukemia, their bone marrow stops working properly. Doctors can sometimes perform a bone marrow transplant, transferring healthy marrow from a donor to help the patient's body start producing healthy blood cells again. These transplants can save lives, which is why bone marrow donors are considered heroes.

The term marrow by itself usually means bone marrow, though it can also refer to the essential core of anything. When someone talks about a story that's funny “right down to the marrow,” they mean it's funny through and through, at its very core, just like the marrow at the core of your bones.