Before writing appeared, knowledge was passed on by speech only. People kept it in their memory, and children heard it from their fathers. This method could lead to forgetting and to change as generations passed. Then, about five thousand years ago, the Sumerians in Mesopotamia invented writing on clay tablets. They used it to record grain and trade. At first these signs were not letters but simple pictures that stood for things. After that, the signs developed into letters, and writing spread among peoples. Thanks to writing, a person could keep his experience in a place outside his mind and pass it on to those who came after him. So the age of written history began.

Question: On what material did the Sumerians first write, according to the text?