What is Bone Marrow? A Kid’s Guide to Bones

What's inside your bones All about bone marrow

Have you ever wondered what’s actually inside your bones? You might think bones are just hard and solid all the way through, like a rock. But that’s not true. Deep inside your bones is something soft, squishy, and seriously hardworking, called bone marrow.

What is bone marrow, exactly?

If you could chop a bone in half, the outside would be hard and smooth, kind of like an eggshell. But the inside would look like a sponge. Bone marrow is the soft material inside your bones.

Marrow has a jelly-like texture, and it’s jam-packed with cells that are busy doing one of the most important jobs in your entire body.

Red marrow versus yellow marrow

Your body has two kinds of bone marrow named after their color:

  • Red marrow is where your body makes new blood cells: red blood cells, white blood cells, and platelets.
  • Yellow marrow is made mostly of fat cells. Your body uses it to store energy, kind of like a backup battery.

When you were born, almost all of your bone marrow was red. As you grow, red marrow slowly turns into yellow marrow. By the time you’re an adult, you’ll have a mix of both.

Why is bone marrow so important?

Bone marrow might be hidden, but it’s extremely important. Your red marrow is basically a blood cell factory, working nonstop to keep your body running. Making blood cells is a big deal because:

  • Red blood cells carry oxygen all around your body, so your brain, muscles, and other organs can work.
  • White blood cells fight off germs and keep you from getting sick.
  • Platelets help your blood clot, so you stop bleeding when you get a cut or scrape.

Without bone marrow, your body wouldn’t be able to make any of these cells.

Where is bone marrow found?

Not every part of every bone is packed with marrow, but it shows up in many places. You’ll find the most marrow in your bigger bones, like your spine, rib cage, hip bones, and femur (that’s your thigh bone, the biggest bone in your whole body).

3 fun facts about bone marrow

  1. You need bone marrow to live.
  2. Your bone marrow makes about 500 billion new blood cells every single day.
  3. You can donate your bone marrow. Doctors can use healthy bone marrow from one person to help save the life of someone with certain diseases. This is called a bone marrow transplant.

See bone marrow for yourself

Bone marrow diagram and science activity for kids - Human Body Learning Lab by Dr. Betty Choi

Want to get a better look at what bone marrow actually looks like?

Try building your own bone marrow model with simple materials from home. It’s one of many fun ways to learn about bones and the skeletal system! Then you can see exactly what’s hiding inside your bones.

Keep exploring the skeletal system!

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READ: Why is the Rib Cage So Important?

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LEARN: What is Cartilage and What Does it Do?

Children's Books About Broken Bones and Strong Skeletons

READ: Books About Broken Bones and Skeleton Science

Human Body Learning has strict sourcing guidelines and relies on information from peer-reviewed research studies, academic institutions, and medical associations.

Published on July 13, 2022. Updated on July 13, 2026 by Betty Choi, MD

Published on July 13, 2022. Updated on July 13, 2026 by Betty Choi, MD

Dr. Betty Choi pediatrician

Betty Choi, MD

Dr. Betty Choi is a Harvard-trained pediatrician who makes learning fun and doable. She created the kids’ anatomy book Human Body Learning Lab, which Science Magazine recommends as a “notable standout in the genre.”