The punctured tire.

One year there were these two guys majoring in Chemistry and
who did very well on all of the quizzes, midterms and labs, etc.,
such that going into the final they had a solid A. These two
friends were so confident going into the final that the weekend
before finals week(even though the Chem final was on Monday),
they decided to go up to Montreal and party with some friends.

So they did. And they had a GREAT time! However, with their
hangovers, they overslept and didn't make it back to campus
until early Monday morning. Rather than taking the final then,
what they did was to find the professor after the final and explain
to him why they missed it.
They told him that they went up to Montreal for the weekend, and
had planned to come back in time to study, but that they had a
flat tire on the way back and didn't have a spare and couldn't get
help for a long time and, therefore, were late getting back to
campus. The professor thought this over and then agreed that
they could make up the final on the following day. The two guys
were elated and relieved.

So, they studied that night and went in the next day at the
scheduled time. The professor placed them in separate rooms
and handed each of them a test booklet and told them to begin.
Each student looked at the first problem, which was something
simple about molarity and the solution was worth 5 points.
"Cool", they thought, "this is going to be easy." They finished that
problem and went on to the next. As they turned the page, they
were unprepared, however, for what they saw. It said:

(95 points) Which tire?