A marvellous answer

A mechanic was removing  the cylinder heads from the motor of a car when he  spotted the famous heart surgeon in his shop, who was standing off to the side, waiting for the service manager to come to take a look at his car. The mechanic shouted across the  garage," Hello Doctor!! Please come over here for a minute." The  famous surgeon, a bit surprised, walked over to the mechanic. The  mechanic straightened up, wiped his hands on a rag and asked argumentatively, "So doctor, look at this.  I  also open hearts, take valves out, grind 'em, put in new parts, and  when I finish this will work as a new one...  So how come you get the big money, when you and me is doing basically the same work? " The  doctor leaned over and whispered to the mechanic.....     " Try to do it when the Engine is RUNNING  "