A professor tells his class that he will give them a surprise examination sometime next week.
For our purposes, an examination is considered a "surprise" if on that day, the class is not 100% sure that they will get an examination. (If they only suspect it, the examination can still be a surprise.)
Having given the class this warning, is it still possible for the professor to give them a surprise examination next week? (Assume that he will not give them an exam if it will not be a surprise.)
The act of telling his class that he will give them a surprise examination sometime next week does not guarantee that he will. Any truly logical student would deduce that the professor's word is not 100% accurate. After all, he could become ill and cancel the week's classes.
Therefore, having the exam on any one day is a "surprise" given the definition above.
Flaw
