A group of mathematicians and a group of physicists are going to a convention on the same train.

The mathematicians get only one ticket for the entire group. What they do is, they pay attention during the journey, and when the inspector starts asking for tickets, before he sees them, they all run to the train's bathroom and lock themselves in, so when the inspector knocks on the door asking for a ticket, they slide under the bathroom door the one they have.

On the way back, the mathematicians don't get any tickets at all. They don't even pay any attention for the inspector. When the inspector starts asking for tickets, all the physicists, who have purchased a single ticket for the entire group this time, run to the train's bathroom and lock themselves in before he sees them. One of the mathematicians knocks on the door and says "ticket please", so the physicists slide it under. Then all the mathematicians run to the next bathroom before the inspector sees them and lock themselves in.