So there's a big mathematical physics conference in London, and all the grad students from the University of Leeds maths (UKism for math) and physics departments all travel down to attend.

The physicists all queue up and get their tickets. The maths students buy one ticket between them. The physics students all point and laugh, of course.

Sure enough, once they're moving down the track the guard (UKism for conductor) comes down the aisle checking tickets. The mathematicians all saunter down to the next car and pile into the loo. When he gets down to it, the guard knocks on the door: "Tickets, please!". They slide their ticket under the door, and the guard punches it and slides it back.

Everyone has a grand time at the conference. Heading back, the physicists (having learned a thing on the way down) get one ticket between them. The mathematicians don't buy any tickets at all.

Sure enough, on the way back north, the guard comes down the aisle.

All the physicists pile into the loo.

One of the maths students knocks on the door and says, "Tickets please!".