A wheel is a circular component that is intended to rotate on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be moved easily facilitating movement or transportation while supporting a load, or performing labor in machines. Wheels are also used for other purposes, such as a ship's wheel, steering wheel, potter's wheel, and flywheel. Common examples are found in transport applications. A wheel reduces friction by facilitating motion by rolling together with the use of axles. In order for wheels to rotate, a moment needs to be applied to the wheel about its axis, either by way of gravity or by the application of another external force or torque. Using the wheel, Sumerians invented a device that spins clay as a potter shapes it into the desired object. Terminology The English word wheel comes from the Old English word , from Proto-Germanic , from Proto-Indo-European , an extended form of the root "to revolve, move around". Cognates within Indo-European include Icelandic "wheel, tyre", Greek , and Sanskrit , the last two both meaning "circle" or "wheel". History The place and time of the invention of the wheel remains unclear, because the oldest hints do not guarantee the existence of real wheeled transport, or are dated with too much scatter. Mesopotamian civilization is credited with the invention of the wheel. However, unlike other breakthrough inventions, the wheel cannot be attributed to a single nor several inventors. Evidence of early usage of wheeled carts has been found across the Middle East, in Europe, Eastern Europe, India and China. It is not known whether Chinese, Indians and Europeans invented the wheel independently or not. The invention of the solid wooden disk wheel falls into the late Neolithic, and may be seen in conjunction with other technological advances that gave rise to the early Bronze Age. This implies the passage of several wheelless millennia even after the invention of agriculture and of pottery, during the Aceramic Neolithic. 4500–3300 BCE (Copper Age): invention of the potter's wheel; earliest solid wooden wheels (disks with a hole for the axle); earliest wheeled vehicles; domestication of the horse 3300–2200 BCE (Early Bronze Age) 2200–1550 BCE (Middle Bronze Age): invention of the spoked wheel and the chariot The Halaf culture of 6500–5100 BCE is