A mechanical engineer, an electrical engineer, a chemical engineer and a software engineer were setting off on a car journey. Their car refused to start.
"I think the problem is the valve gear", said the mechanical engineer. "Probably the cam belt has jumped a cog. Let's take a look under the camshaft cover".
"I'm sure you'll find that the ignition is at fault", said the electrical engineer. "Over 90% of car failures are due to electrical problems. We should check all the connections and make sure that the ignition leads are dry".
"Don't overlook the fuel system", suggested the chemical engineer. "There could be water in the petrol, maybe we should drain the tank".
The software engineer was silent for a moment. "Let's all get out of the car, then get back in again and see if it goes".