Two groups of students - math and engineering majors - boarded a train that was headed for a technical convention. Each of the math majors had a ticket, but their engineering counterparts had only one ticket between them.
The math majors were snickering at this when an engineering student shouted, "Here comes the conductor!" With that, all the engineering majors squeezed into a bathroom. The puzzled math students watched as the conductor collected their tickets, then knocked on the bathroom door and said, "Ticket please." The conductor took the single ticket that was passed under the door and left.
Not to be outdone, the math students boarded the returning train with only one ticket, and again they laughed because this time their rivals had no ticket at all.
When the engineering lookout yelled, "Conductor coming!" all the engineers crowded into one bathroom, while the math majors piled into another. Then, before the conductor entered the car, one of the engineers came out of his bathroom and knocked on the math majors' door.
"Ticket please," he said.