The skull is made of bone, but the stuff flaking off of it is a compound that is made to self-ignite in light. Thaumaturgical, and it tends to explode if you pack a lot of it and expose it to light.<br/><br/>Frost, though gives a more joking answer. "Well, it seems to be devoid of flesh and not attached to a neck. Combine that with being empty, and it's a horrible, horrible excuse for a head. It can't do anything you expect of a head. Not even a headbutt."