Originally Posted by Duodenum
A medical scan revealed that while Hagler's skull was normal, the band of muscle which surrounded his skull was one inch thick, where the average person has a layer one quarter inch thick. Certainly, we've all seen performing martial artists who were able to break planks and cinderblocks with their heads, so it's self evident that one can be conditioned to do this. The small muscles of the head can be reportedly developed somewhat through repeated contact with punches in sparring.
I hope you intended this as a joke, but forgot to warn the others.