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If you’ve ever seen a house being built, you’ll recognize the wooden framework as the beginning of the structure. Just as this framework provides the structure for the insulation, walls and roof of a house, our bones are also a strong framework. Bones provide a firm support system that protects our internal organs and give our muscles something to attach to. If bones are the frame, then fat is our body’s insulation and skin is our walls. Just like insulation and walls protect the inner workings of our homes, bones, fat and skin protect our nerves, blood vessels and internal organs. Our skull protects the brain—like a roof—and our rib cage shields the heart and lungs like the walls of our house protect us from the outdoors.

Bones have a solid outside, called compact bone, that provides strength and covers the inner spongy cavity called bone marrow. Compact bone is hard, white and smooth. It gets its strength from the calcium that it stores. Bone marrow—which manufactures the white blood cells that form our immune system and the red blood cells that carry oxygen to all of our organs—is like a sponge with lots of little air pockets. These pockets reduce the weight of our bones. If they were completely solid, they would be so heavy we wouldn’t be able to walk or run.

Our bodies are designed to move, so our bones are separated by joints and cartilage that lets us bend, run, jump and dance. Cartilage, though extremely strong, isn’t as hard as bone or as flexible as muscle. We have cartilage between the bones of our ribs and spine, in the joints of our arms and legs and in our ears and nose. It acts as a shock-absorber and keeps our bones from rubbing against each other, which can be very painful. Cartilage is the only tissue in our body that doesn’t have blood vessels, making cartilage one of the slowest tissues to grow and heal. This is why recovering from joint injuries takes a long time.

Weight bearing activities and strength training exercises can help keep our bones strong. Because compact bone is the main place calcium is stored in our bodies, it’s important to eat enough calcium to build strong bones, especially in early life. Up to 90 percent of peak bone building happens by age 18 in girls and 20 in boys. If we don’t get enough calcium, we may be more likely to break our bones or develop osteoporosis later in life.

So, like our homes, our bones also need regular maintenance. Keep your own framework healthy by exercising and eating bone-building, calcium-rich foods.

Kimberly A. Huey, PhD, FAPS, FACSM, is professor of physiology in the Department of Health Sciences at Drake University in Des Moines, Iowa. Her research focuses on contractile and cellular adaptations in skeletal muscle to changes in loading and activation such as exercise or disuse as well as the effects of medications on muscle function.