A math/engineering convention was being held. On the train to the
convention, there were a bunch of math majors and a bunch of engineering
majors. Each of the math majors had his/her train ticket. The group of
engineers had only ONE ticket for all of them. The math majors started
laughing and snickering.

Then, one of the engineers said "here comes the conductor" and
then all of the engineers went into the bathroom. The math majors
were puzzled. The conductor came aboard and said "tickets please"
and got tickets from all the math majors. He then went to the
bathroom and knocked on the door and said "ticket please" and the
engineers stuck the ticket under the door. The conductor took it
and then the engineers came out of the bathroom a few minutes later.
The math majors felt really stupid.

So, on the way back from the convention, the group of math majors
had one ticket for the group. They started snickering at the
engineers, for the whole group had no tickets amongst them. Then,
the engineer lookout said "Conductor coming!". All the engineers
went to one bathroom. All the math majors went to another bathroom.
Then, before the conductor came on board, one of the engineers left
the bathroom, knocked on the other bathroom, and said "ticket please."