Many tools are based upon the principle of the lever. A lever is a rigid bar, straight or bent, free to turn about a point called a fulcrum. Levers are generally divided into three kinds or classes, the class being determined by the position of the fulcrum in relation to the applied force or effort and the resisting force, i.e., the weight. The mechanical principle of the lever was discovered by a Greek named Archimedes, who lived in the third century. He stated that if he had a lever long enough and a place to stand, he could move the earth.