Which is the longest bone in the human body?

The femur, running from hip to knee, at roughly a quarter of your height - so it can be used in reverse, and forensic scientists estimate a person's stature from a single thigh bone with surprising accuracy.

It is also the strongest thing in your body, able to take something like thirty times body weight in compression, which is why breaking one takes real violence. It is not solid: the interior is a lattice of struts arranged along the exact lines that the loads travel, so it is enormously strong for its weight. The pattern rearranges itself if the load changes, which is why bone thins in the weightlessness of orbit.

The head of the femur has a poor blood supply arriving through a narrow route, which is why a fractured hip in an older person is so serious - the break can cut off the supply and the bone dies. That single injury remains one of the commonest reasons an independent older person stops being independent.