On the train to a math and engineering convention, a group of math majors and a group of engineering majors sat in the same car. Each of the math majors had his/her train ticket, but it became clear that the group of engineers had only ONE ticket amongst them. The math majors started laughing and snickering.

When one of the engineers said “Here comes the conductor”, all of the engineers went into the bathroom. The math majors were puzzled. The conductor came aboard, said “tickets please” and collected tickets from all the math majors. He then went to the bathroom, knocked on the door and said “ticket please” and the engineers stuck the ticket under the door. The conductor took it and left, and 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 this time the whole group had NO tickets amongst them.
When the engineer lookout said “Conductor coming!”, all the engineers went to one bathroom. All the math majors went to another bathroom. Just before the conductor came on board, one of the engineers left the bathroom, knocked on the other bathroom, and said “ticket please.”