Every good student knows enough to give exam answers one last look.
Consider the fellow who sits down for a true-false exam, puzzles over it for a few minutes, then pulls a coin out of his pocket. He flips it, marks the exam, flips it again, makes another mark and continues flipping until he answers all the questions. The student finishes in about 30 minutes while the rest of the class is still sweating it out. The instructor watches him go through the exercise, thinking the law of averages would dictate that the student should get half the answers correct. Then he is surprised to see the fellow start to flip the coin again, this time furiously.