I realise this is related to biology, but I think the question would be better suited to those who study or have studied physics. The question is this: I learned recently that the cavity in bones, actually increases the overall strength of the bone, even though it has less mass than if it were filled. I was wondering why this is so. Apparently, if you have two glass rods of equal diameter, one of which has a cavity (like a bone), and the other is completely solid throughout, then the rod with the cavity is the stronger of the two. This can be demonstrated by securing them and gradually increasing the weight on the middle of their lengths. The tube with the cavity is stronger by far, according to my biology teacher. Why does the cavity increase the strength?