A math/computer science convention was being held. On the train to the convention,

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

Then, one of the CS majors said "here comes the conductor" and then
all of the CS majors 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 CS majors stuck
the ticket under the door. The conductor took it and then the CS
majors 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 CS majors,
for the whole group had no tickets amongst them. Then, the CS major
lookout said "Conductor coming!". All the CS majors went to the
bathroom. All the math majors went to another bathroom. Then, before
the conductor came on board, one of the CS majors left the bathroom,
knocked on the other bathroom, and said "ticket please."