Three engineers and three mathematicians are traveling by train to a conference. At the station, the three mathematicians each buy tickets and watch as the three engineers buy only a single ticket. "How are three people going to travel on only one ticket?" asks a mathematician.
"Watch and you'll see," answers an engineer.
They all board the train. The mathematicians take their respective seats, but all three engineers cram into a restroom and close the door behind them. Shortly after the train has departed, the conductor comes around collecting tickets. He knocks on the restroom door and says,"Ticket, please." The door opens just a crack and a single arm emerges with a ticket in hand. The conductor takes it and moves on. The mathematicians see this and agree it is quite a clever idea.
So after the conference, the mathematicians decide to copy the engineers on the return trip and save some money (being clever with money, and all that). When they get to the station, they buy a single ticket for the return trip. To their astonishment, the engineers don't buy a ticket at all.
"How are you going to travel without a ticket?" asks one perplexed mathematician.
"Watch and you'll see," answers a engineer.
When they board the train, the three mathematicians cram into one restroom and the three engineers cram into another one nearby. The train departs. Shortly afterward, one of the engineers leaves his restroom and walks over to the restroom where the mathematicians are hiding. He knocks on the door and says, "Ticket, please."