Difference between a mechanic and a surgeon

A mechanic was removing a cylinder head from the motor of a Harley motorcycle when he spotted a well-known heart surgeon in his shop.
The surgeon was there, waiting for the service manager to come and take a look at his bike.
The mechanic shouted across the garage, "Hey, Doc, can I ask you a question?"

The surgeon a bit surprised, walked over to the mechanic working on the motorcycle. The mechanic straightened up, wiped his hands on a rag and asked, "So Doc, look at this engine. I open its heart, take valves out, fix 'em, put 'em back in, and when I finish, it works just like new. So how come I get such a small salary and you get the really big bucks, when you and I are doing basically the same work?"

The surgeon paused, smiled and leaned over, and whispered to the mechanic... 
"Try doing it with the engine running."


Source: Unknown with thanks!