Two college seniors had a week of exams coming up. However, they decided to party instead. So, when they went to the test, they decided to tell the professor that their car had broken down the night before due to a very flat tyre and they needed a bit more time to study.
The professor told them that they could have another day to study.

That evening, both of the boys crammed all night until they were sure that they knew just about everything.

Arriving to class the next morning, each boy was told to go to separate classrooms to take the exam. Each shrugged and went to two different parts of the building.
As each sat down, they read the first question. "For 5 points, explain the contents of an atom." At this point, they both thought that this was going to be a piece of cake, and answered the question with ease.

Then, the test continued... "For 95 points, tell me which tyre it was."