The first books were quite different from the books of today. They were made of baked clay tablets. Some of these tablets that have been found were used in Mesopotamia about fifty-five hundred years ago. The people of that time used symbols to represent their language. When the clay was soft, the symbols were written in the clay. After the tablets were baked, the clay hardened and the messages were permanently preserved. Question： The oldest books found were made of _______ .