5 engineers and 5 mathematicians need to travel to a distant town.
They all meet up at the train station. They go to the ticket office. The mathematicians buy 5 tickets. And then watch in astonishment as the engineers buy just one tickets.
The mathematicians ask the engineers how this could work out. The engineers reply: "wait and see!".

They all get on the train. After some time one of the engineers spots the conductor approaching, so he informs the other engineers. They all head for the toilet and get in.

The conductor checks the mathematicians' tickets, then proceeds, stopping at the toilet noticing the "occupied" sign. So he knocks on the door, calls "Ticket please"! One of the engineers pushes the ticket outside at the bottom of the door. The conductor checks it, pushes the ticket back inside with a "Thank you!".

The mathematicians are very impressed by the engineers' cleverness.

So a few days later, they all gather again at the train station for going back home. The mathematicians remember what the engineers did for tickets a few days ago and now buy just one ticket, and are very surprised as the engineers do not buy a ticket at all!

The mathematicians ask the engineers how this could work out. The
engineers reply: "wait and see!".

Again they are in the train, and one of the mathematicians spots the approaching conductor. So the engineers vanish in one toilet, the mathematicians in the other. Just after the last mathematicians disappeared, one of the engineers comes back out of the engineers' toilet, goes to the mathematicians toilets, knocks and says "Ticket please!"