The first invention of human beings was the wheel. Although no wheel forms are found in nature, undoubtedly the earliest "wheels" were smooth logs which were used for moving weights over the earth's surface. No one recorded who he or she was or when it happened, but when the "first inventor" placed a wheel on an axle, people began to roll from one place to another. Records of this type of wheel have been found among Egyptian relics dating back to 2,000 B.C. and earlier Chinese civilizations are credited with independent invention of the same mechanism. The wheel so fascinated the human mind that people have spent centuries building machines around it; yet in over 4,000 years its basic design has remained unchanged. All about us we see the spinning shafts, gears, flywheels, pulleys, and rotors which are the descendents of the first wheel. The roaring propeller of an aircraft engine, the whirling wheel of a giant steam turbine, and the hairspring of a tiny watch are examples of the rotary motion which characterizes our mechanical world. It is hard to conceive of continuous motion without the wheel. Select the statement that best expresses the main idea of the paragraph.