Your skeleton is almost brand new

The skeleton that holds up your body isn't a fixed, dead structure: it's living tissue that is torn down and rebuilt without pause. It's estimated that an adult renews their entire skeleton roughly every ten years.

Inside the bone, two opposing teams are at work: some cells remove old bone and others lay down new bone. That back-and-forth repairs microfractures, adjusts the shape to the strain, and releases minerals when the body needs them.

That's why weight-bearing exercise strengthens bones and a lack of movement weakens them: the skeleton adapts to what you ask of it. In a sense, your bones are a building under permanent construction.

Source: U.S. National Institutes of Health (NIH)