"Night Shift" joke

During work Mike and John are chatting,
Mike: "I've been taking night courses for five months now, and I have an exam next week."
John: "Oh!"
Mike: "For example, do you know who Graham Bell is?"
John: "No."
Mike: "He is the inventor of the phone in 1876. If you take night courses you would know this."
The next day the same discussiontook place:
Mike: "Do you know who Alexander Dumas is?"
John: "No."
Mike: "He is the author of "The 3 Musketeers". If you took night courses you would know this."
The next day, once again:
Mike: "And do you know who Jean Jacques Rousseau is?"
John: "No."
Mike: "He's the author of "Confessions", if you took night courses you would know this."
Now this time John got irritated and said, "And do you know who Steven Turner is?"
Mike: "No."
John: "He is the guy sleeping with your wife! If you stopped night duties you would know this!"