How did the ancient Sumerians write on clay tablets?

Long before paper, before pens, before anyone had even dreamed of a notebook, the Sumerians of ancient Mesopotamia had a brilliant idea. They looked down at the riverbank mud beneath their feet and thought, "What if we could write on THAT?" And so they did. This is the story of how the world's first writing was pressed into wet clay.
First, they needed something to write ON. They scooped up smooth clay from the river and patted it into a small flat tablet, about the size of your palm. While it was still soft and damp, it behaved a bit like fresh dough — squishy enough to take a mark, firm enough to hold its shape. The clock was now ticking, because clay does not stay soft forever.
