Many inventions which seem relatively recent in fact date back thousands of years. This encyclopaedia focuses on the applied science of the Ancient World. Those ancient inventions with a modern sound include: plastic surgery performed in India in the 1st century BC; an early form of the computer used by the ancient Greeks; the postal service of Ancient Baghdad; calendars used in France as early as 13,000 BC; steam engines in Ancient Greece; and Aztec chewing gum.