What specific features of the composition and structure of bones provide their flexibility, strength and relative lightness?

Bones have a porous structure that makes them lightweight, while they are not devoid of strength. This is achieved due to the trabecular or spongy structure. Many bridges form a strong frame, the gaps between which are filled with air. Flexible bones, lacking strength, developed mainly in childhood. In their structure, the main mass is cartilaginous tissue, which makes them resistant to fractures with excessive bending and stretching.