A group of three biologists

from a local university observe a group of three mathematicians (from the same university) line up behind them to wait for a train ride to an academic conference. The biologists purchase three tickets, but the mathematicians only purchase one. Intrigued, the biologists board the train and take a compartment adjacent to the mathematicians, monitoring their actions closely.

Right as the conductor stops by the biologists' compartment, they catch a glimpse of the mathematicians sneaking off to the bathroom. After the conductor checks the three biologists' tickets, he knocks on the bathroom door. A single ticket slides underneath the door. The conductor examines the ticket, punches a hole in it, and slides it back underneath.

The biologists are amused, and a little miffed. The conference ends three days later, and soon it's time to board the train back home again. The biologists purchase only one ticket. However, this time the mathematicians purchase none. This excites the biologists - they can't wait to see the mathematicians thrown off the train.

As the conductor starts coming down the hall, the biologists file into the empty bathroom. Minutes later, they hear a knock on their door. One of the biologists coughs and slides the ticket underneath. The group waits for their ticket to return, but all they hear is the sound of footsteps shuffling away.