Engineers on a train
Three engineers and three mathematicians are on a train going to a conference. The mathematicians each bought a ticket. The engineers have one between them. As the conductor starts through the train car, the engineers all rush off and jump into the small lavatory.
The conductor knocks on the door of the lavatory and says “Ticket, please.” At which point the engineers slide the one ticket through a ventilation slot and the conductor punches it. The mathematicians think this looks like a good trick and decide to try it on the train ride back home.
As the mathematicians board the train they have one ticket between them. The engineers have no ticket!
After a while, one of the engineers says, “Here comes the conductor!” So all three mathematicians jump up and run into the lavatory with their one ticket.
One of the engineers goes to the lavatory door and says “Ticket, please.”