Some of the contributions were things like school and cuneiform, which is wedged shape writing in clay. This made possible the first literature, poetry, letters, bills, and accounting systems, they also invented the wheel, the first wheeled vehicles and many of the farm tools that are still used such as hoes, shovels, and rakes. The Phoenicians created the first alphabet. Some of their letters are still used today, though somewhat changed in form. The Mesopotamians also made ziggurats, which were sacred 'mountain' temples to their gods. Egyptians later borrowed the idea of step-shaped buildings and converted them into pyramids. Ancient Mesopotamians invented the system by which we still keep time using base 20. They devised the twelve month calendar which we still use. The calendar they developed used constellations to tell time, from this study of the stars came astrology. They developed the first mathematics and geometry. King Hammurabi is credited with the idea of written laws. Some Mesopotamians began to worship one god instead of many, a belief that is central to Judaism, Christianity, and Islam. They built two of the wonders of the ancient world: the Hanging Gardens of Babylon and the royal tomb at Halicarnassus. The Hittites were the first to make and use iron weapons which revolutionized warfare.