A math/engineering convention was being held. On the train to the convention, there were a bunch of mathematicians and a bunch of engineers. Each of the mathematicians had his/her train ticket. The group of engineers had only ONE ticket for all of them. The mathematicians started laughing and snickering. Then, one of the engineers said "here comes the conductor" and then all of the engineers went into the toilet. The mathematicians were puzzled. The conductor came aboard and said "tickets please" and got tickets from all the mathematicians. He then went to the toilet and knocked on the door and said "ticket please" and the engineers stuck the ticket under the door. The conductor took it and then the engineers came out of the toilet a few minutes later. The mathematicians felt really stupid.
So, on the way back from the convention, the group of mathematicians had one ticket for the group. They started snickering at the engineers, for the whole group had no tickets amongst them. Then, the engineer lookout said "Conductor coming!". All the engineers went to one toilet. All the mathematicians went to another toilet. Then, before the conductor came on board, one of the engineers left the toilet, knocked on the other toilet, and said "ticket please."