ויאמר ה’ כבד לב פרעה…(וארא ז:יד)

And Hashem told Moshe that Pharaoh’s heart became hard…

Pharaoh’s heart had the nature of כבד – a liver.  The nature for all parts of meat is that the more you cook it, the softer the meat.  The liver, however, has an adverse reaction.  The more you cook liver, the more complex its texture. So too, was the nature of Pharaoh’s heart – the more Makos (plagues) thrust upon the Egyptians, the harsher his heart.