Mystery of science: The Hardness of a Dog's Head:
The Rockwell scale indicates the hardness of materials by way of an indentation test. Hardened steel, like in a high-end knife, will have a rating of around 55 to 62. Lower grade metals score in the low forties. A hard piece of tempered glass can score 80 or higher.
Now, none of these materials come close in hardness to that of a canis lupis familiaris (dog) head. Paradoxically, a dog's head is comprised of pretty standard organic stuff. You have bone cells, muscles, and even some cushioning fur. Yet somehow, these items combine into diamond-like hardness that is capable of battering the unwary into unconciousness.
Having shattered a diamond with his head, Rufus looks hopefully for other items to headbutt
The evolution of the hard dog head probably came about in response to icy surfaces or the use of hardwood floors. When traction is scarce, the feet scrabble and the head is often used as a crude brake for the dog. Therefore, the hard head has evolved to protect the dog's brain. Still, science cannot explain the unusual hardness properties.