ClipArt ETC
This figure shows a thigh bone that has been softened by exposing it to acid, then twisted in a knot to demonstrate it's flexibility. Bones are composed of a "mineral" or hard substance and an "animal" or soft substance. The animal component can be removed by exposing the bone to extreme heat, causing the bone to become brittle and break easily. Similarly, the mineral component can be removed by exposing the bone to acid. Once the brittle mineral part is removed by the acid, the bone is soft and flexible.

Bone Exposed to Acid and Twisted

This figure shows a thigh bone that has been softened by exposing it to acid, then twisted in a knot…