Three economists and three mathematicians were going for a trip by train. Before the journey, the mathematicians bought 3 tickets but economists only bought one. The mathematicians were glad their stupid colleagues were going to pay a fine.
However, when the conductor was approaching their compartment, all three economists went to the nearest toilet. The conductor, noticing that somebody was in the toilet, knocked on the door. In reply he saw a hand with one ticket. He checked it and the economists saved 2/3 of the ticket price.
The next day, the mathematicians decided to use the same strategy - they bought only one ticket, but economists did not buy tickets at all! When the mathematicians saw the conductor, they hid in the toilet, and when they heard knocking they handed in the ticket. They did not get it back.
Why? The economists took it and went to the other toilet.