"In the late 1970's a balancing machine salesman visited a customer's plant who had just received a new balancer from the salesman's competitor. The plant manager said they were very happy with their automatic balancing machine and offered to show it to the salesman. The manager walked the salesman out on the floor and the two of them watched the operator and balancer in action. The operator placed a part on the balancer and closed the door. The balancer spun up the part@ welded on a weight@ spun up again@ and displayed ""good part."" The operator removed the balanced part@ put in a new part@ and closed the door. The balancer spun up the part@ welded on a weight@ spun up again@ and displayed ""good part."" This scenario was repeated several more times as the salesman and the manager watched. The manager commented@ ""We just love our new machine. All day long it balances parts by welding on weights and puts out good parts."" The salesman suggested having the operator place a ""balanced part"" back in the balancer again just to see what would happen. So the operator placed the previously balanced part back in the balancer again and closed the door. The balancer spun up the part@ welded on a second weight@ spun up again@ and displayed ""good part."" The manager had the operator take another balanced part and put it into the balancer again. Again@ the balancer spun up the part@ welded on another weight@ spun up again@ and displayed ""good part."" Suddenly the manager was not so happy with his balancing machine. It seems this machine was not balancing the parts at all. They had purchased an expensive welding machine to weld weights on their parts."