A young student reports for a final examination that consists of only true/false - type statements.
The student takes a seat in the hall, stares at the question paper for five minutes, removes a coin and starts tossing the coin and marking the answer sheet.
Heads means true, tails means false.
The young student is all done in 20 minutes while the rest of the class is sweating it out.
But, suddenly during the last few minutes, the young student is seen desperately throwing the coin, swearing and sweating.
The teacher, alarmed, approaches the student and asks what is going on.
'Well I finished the exam in half an hour,' says the student, 'but I thought I ought to recheck my answers.'