A group of engineers are on a train across from a group of mathematicians. The man comes by to check their tickets. The engineers only have one ticket for all of them and the mathematicians have tickets for their whole group. The ticket checker gets close to their row when the 10 engineers go to the bathroom. The Ticket checker knocks and they pass the ticket through the door.
On the ride back the mathematicians have one ticket and the engineers have zero. The ticket checker comes and the mathematics run to the bathroom to try and pull off what the engineers did. The engineers then knock on the bathroom door (pretending to be the ticket man) and takes the mathematician’s only ticket. The engineers run to the other bathroom and successfully repeat the con.