Three engineers and three accountants are traveling by train to a conference. At the station, the three accountants 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 an accountant. "Watch and you'll see," answers an engineer. They all board the train. The accountants 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 accountants saw this and agreed it was quite a clever idea. So after the conference, the accountants decide to copy the engineers on the return trip and save some money (being clever with money, and all!) 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?" says one perplexed accountant.
"Watch and you'll see," answers an engineer. When they board the train the three accountants cram into a 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 accountants are hiding. He knocks on the door and says, "ticket, please."