A heart surgeon took his car to his local garage for a regular service, where he usually exchanged a little friendly banter with the owner, a skilled but no especially wealthy mechanic.

"So tell me," says the mechanic, "I've been wondering about what we both do for a living, and how much more you get paid than me..."

"Yes?..." says the surgeon.

"Well look at this," says the mechanic, as he worked on a big complicated engine, "I check how it's running, open it up, fix the valves, and put it all back together so it works as good as new.  We basically do the same job don't we?  And yet you are paid ten times what I am - how do you explain that?"

The surgeon thought for a moment, and smiling gently, replied, "Try it with the engine running."