5 mathematicians and 5 engineer are going to visit another university.

At the railway station, the mathematicians buy 5 tickets, while the engineers only buy a single ticket. The mathematicians are surprised, and ask the engineers how this will work out.

The engineers tell the mathematicians: wait and see.

They all enter the train (here in Germany, you don't need to show tickets before entering the train, the conductor will check them while the train is moving).

After a while, the conductor is approaching to check the passengers' tickets.

The five engineers all cram into the waggon's (or is it carriages's - sorry, my English is not the best) toilet and lock themselves in.

The conductor, after checking the mathematicians' tickets, notices that the toilet is in use, and knocks on the toilet door - one of the engineers says: "sorry, I will need some more time in here" and pushes the ticket outside through the narrow slit at the bottom of the door.

So they reach the other university, visit their colleagues, and start for the return journey.

This times - having learned from the engineers - the mathematicians buy only one ticket. And they are surprised when they notice that the engineers this time buy no ticket at all.

Again they ask how this will work, and get the same answer: "wait and see!".

And again they enter the train. When the conductor is approaching, the mathematicians cram into the toilet at the one end of the waggon, while the engineers cram into the toilet at the other end of the waggon.

When all the mathematicians are locked away, one of the engineers slips out again, runs to the other toilet, knocks on the door and says: "ticket please" ...