Invention of the wheel
The wheel was invented in the 4th millennium BC in Lower Mesopotamia(modern-day Iraq), where the Sumerian people inserted rotating axles into solid discs of wood. It was only in 2000 BC that the discs began to be hollowed out to make a lighter wheel. This innovation led to major advances in two main areas. First, transport: the wheel began to be used on carts and battle chariots. Second, and more importantly, it contributed to the mechanisation of agriculture (animal traction, crop irrigation) and craft industries (for example, the centrifugal force of the wheel is the basic mechanism in windmills).