What is a wheel?

A wheel is a circular block of a hard and durable material at whose center has been bored a hole through which is placed an axle bearing about which the wheel rotates when torque is applied to the wheel about its axis. 

Who and when the wheel was invented?

The wheel was invented in the 4th century 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.

What makes a wheel sometimes look as it is going backwards?

Sometimes when we are watching a film of a wheeled vehicle, especially of one with a spoked wheel, it seems to be going backwards instead of forwards as the vehicle slows down. This is because the film is made up of a series of individual pictures taken in quick succession. Our eyes blend the individual images together and we see a smooth movement rather than a series of still pictures. If the eye does not blend the images correctly, the wheel seems to turn backwards.

This effect can also sometimes be obtained by looking at a spoked vehicle through railings.