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The Surprise Exam

It is Friday afternoon, and the teacher says: "next week there will be a surprise test, I will not tell you on which day of the week, I promise you that it will be a total surprise to everyone but me".

After a few seconds George suddenly start laughing. The teacher, a bit perplexed, asks George to please explain himself.

"Well", George start, "according to your rule we will have no test next week. After all, it cannot be on Friday, as on Friday morning we would know that it must be on Friday, and therefore it would not be a surprise test! The test cannot be on Thursday either: we already know that it cannot be on Friday, so on Thursday morning we would expect the test and it would not be a surprise".

George continues: "With the same argument we can eliminate Wednesday, Tuesday and Monday!"

The teacher smiles, wishes everyone a good weekend and leaves.

The week starts, Monday, Tuesday and Wednesday pass without test. On Thursday the teacher suddenly asks everyone to put away all books, take out paper and pen and announces the start of the test!

It turns out that not only was George wrong and there was a test, but George actually was completely taken by surprise! So the teacher did keep his promise!

What is wrong with Georges reasoning?

Unfortunately I cannot remember when and where I heard or read this problem. If anyone knows a publication please let me know.

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