L. Frank Baum said this quote

“ The Glass Cat is one of the most curious creatures in all Oz. It was made by a famous magician named Dr. Pipt before Ozma had forbidden her subjects to work magic. Dr. Pipt had made the Glass Cat to catch mice, but the Cat refused to catch mice and was considered more curious than useful.This astonishing cat was made all of glass and was so clear and transparent that you could see through it as easily as through a window. In the top of its head, however, was a mass of delicate pink balls which looked like jewels but were intended for brains. It had a heart made of a blood-red ruby. The eyes were two large emeralds. But, aside from these colors, all the rest of the animal was of clear glass, and it had a spun-glass tail that was really beautiful. ”