A group of mathematicians and a group of engineers are traveling together by train to attend a conference on mathematical methods in engineering. Each engineer has a ticket whereas only one of the mathematicians has one. Of course, the engineers laugh at the unworldly mathematicians and look forward to the moment the conductor shows up.
Suddenly one of the mathematicians shouts: "Conductor coming!" All the mathematicians disappear into one washroom. The conductor checks the ticket of each engineer and then knocks at the washroom door: "Your ticket, please." The mathematicians stick the one ticket they have under the door, the conductor checks it and leaves. A few minutes later, when it is safe, the mathematicians come out of the washroom. The engineers are impressed.
When the conference has come to an end, the engineers decide that they are at least as smart as the mathematicians and also buy just one ticket for the whole group. This time the mathematicians have no ticket at all...
Again one of the mathematicians shouts: "Conductor coming!".
All the engineers rush off to one washroom. One of the mathematicians goes to that washroom, knocks at the door, and says: "Your ticket, please..."
If you like that, there are more excellent math jokes at University of Alberta's math professor Volker Runde's website: http://www.math.ualberta.ca/%7Erunde/jokes.html