A Cardiologist took his car into the shop for work on the engine.
The mechanic said, "Doc we have similarities in our job, you bring your car to me to work on the engine which is the heart of the car. I open her up, take out the faulty valves, put new ones, closer up and she runs like new. So how come you make the big bucks and I don't?"
The Doc paused for a minute searching for an answer, then replied "Try doing that with the engine running!"