What's the difference between a mechanic and a cardiologist?

A mechanic was removing a cylinder head from the motor of a car when he spotted a well-known cardiologist in his shop.
The cardiologist was there waiting for the service manager to come and take a look at his car when the mechanic shouted across the garage, “hey doc, want to take a look at this?”
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The cardiologist, a bit surprised, walked over to where the mechanic was working. The mechanic straightened up, wiped his hands on a rag and asked,
“So doc, look at this engine. I opened its heart, took the valves out, repaired or replaced anything damaged, and then put everything back in, and when I finished, it worked just like new. So how is it that I make $48,000 a year and you make $1.7 million, when you and I are doing basically the same work?”
The cardiologist paused, leaned over and whispered to the mechanic…
“Try doing it with the engine running”.
Thanks to Fran  for sending this in. If you want to read some other great jokes, click here. Or to submit one of your own to share with the Starts at 60 community, click here.