The wheel is a basic but highly important simple machine which has helped mankind in countless ways. It's a round circular shape which rotates around its center and has many applications, particularly in mechanical engineering. When it is built around an axle, it can support weight allowing humans to carry heavy objects over vast distances with little expenditure of energy. Wheels are used in vehicles such as cars and trains and airplanes (when landing).
By rolling, the round surface of the wheel maintains continuous contact with a flat plane and converts rotational energy into linear movement. Friction is an important limiting factor in energy transfer.
Jared Diamond in his book Guns, Germs and Steel wondered why civilizations in the Americas had not invented the wheel, while peoples from the Eurasian continent had. He speculated that the east-west axis of geography which characterizes the continents of Europe, Asia and North Africa meant that more and different species of plants and animals were in competition with each other, which enabled humans to take advantage of a greater variety of foods, and breed a wider variety of animals. For example, the Eurasian landmass had horses, while the Americas did not (until Europeans introduced them following Columbus). Since the climate from western Spain to Japan was roughly similar, it was possible for different societies to learn from each other via warfare as well as trade, facilitating the growth of knowledge. In contrast, the American hemispheres had a north-south axis, argued Diamond, which meant there were radical changes in climate from North America through Central America and down to South America, which stifled commerce and competition between animals and tribes of people. When the two civilizations came into contact following 1492 with the discovery by Christopher Columbus, the Europeans not only had inventions such as the wheel, but writing, guns, steel and a technically advantaged civilization.