A blonde reports for her university final exam which consists of mainly true and false questions. 

She takes her seat in the examination hall, stares at the question paper for five minutes, and then in a fit of inspiration takes her purse out, removes a coin and starts tossing the coin and marking the answer sheet: true for heads and false for tails. 

Within thirty minutes she is all done, whereas the rest of the class is still working furiously.

During the last few minutes, she is seen desperately throwing the coin, swearing and sweating. 

The moderator, alarmed, approaches her and asks what is happening.

"I finished the exam in a half hour," she replies. "Now I'm rechecking my answers."