The transition from an oral culture, in which knowledge was handed down through stories, songs, and apprenticeships, to a literate one, based on the written word, was held back for centuries by the lack of suitable writing material. Stone and clay tablets were used, but they were prone to fracture and were bulky and heavy to transport. Wood suffers from splitting and is susceptible to decay. Wall paintings are static and space is limited. The invention of paper, said to be one of the four great inventions of the Chinese, solved these problems, but it wasn't until the Romans replaced the scroll with the codex ─ or, as we call it now, the book ─ that the material reached its full potential. That was two thousand years ago, and it is still a dominant form of the written word. That paper, a much softer material than either stone or wood, won out as the guardian of the written word is a remarkable materials story.