Joke : What's the difference

A mechanic was removing a cylinder-head from the motor of an MG when he spotted a heart surgeon in his shop.
The surgeon was there waiting for the service manager to come take a look at his car when the mechanic shouted across the garage, "Hey Doc, want to take a look at this?"

The surgeon, a bit surprised, walked over to where the mechanic was working on the sports car. The mechanic straightened up, wiped his hands on a rag and asked, "So Doc, look at this engine. I open its heart, take the valves out, repair any damage and then put them back in and when I finish, it works just like new. So how come I make $40000 a year and you get $150000 when you and I are doing basically the same work?"
The surgeon paused, smiled and leaned over, then whispered to the mechanic...

"Try doing it with the engine running."