One night four students were partying and did not study for an exam the next day.  In the morning they thought of a plan.

They went to the teacher and said they had been at a wedding the night before and on their way home they got a puncture in their tire.  It took so long to fix they were in no condition to take the exam.

The teacher kindly allowed them an extra day to do a re-test.  The students all thanked him and said they'd be ready.

When they took their test they were required to sit in separate classrooms.  The test consisted of one question, worth 100% of the semester grade.

The question....

Which tire was flat?

Let the moral of the story guide you for the rest of the day.