Why is Doctor paid more than a mechanic?

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..... . . . . . . . ... .. .. Doctor said : " Try to do it when the Engine is RUNNING "