Three engineers and three mathematicians

  Three engineers and three mathematicians are travelling by train to a
  conference. At the station, the three mathematicians each buy tickets
  and watch as the three engineers buy only a single ticket.  "How are
  three people going to travel on only one ticket?" asks a
  mathematician.  "Watch and see," replies an engineer.  They all board
  the train.  The mathematicians take their respective seats, but all
  three engineers cram into a rest room and close the door. Shortly
  after the train departs, the conductor comes around collecting
  tickets.  He knocks on the rest room door and says, "Ticket, please."
  The door opens just a crack and a single arm emerges with a ticket in
  hand.  The conductor takes it and moves on.  The mathematicians see
  this and agree it is quite a clever idea.  After the conference, the
  mathematicians decide to copy the engineers on the return trip and
  save some money.  They buy a single ticket for the return trip, but
  are astonished to see that the engineers don't buy any ticket at all.
  "How are you going to travel without a ticket?" asks one perplexed
  mathematician.  "Watch and see" is the answer.  They board the train,
  the three mathematicians cram into one rest room and the three
  engineers cram into another one nearby.  Shortly after the train
  departs, one of the engineers leaves his rest room and walks over to
  the rest room where the mathematicians are hiding.  He knocks on the
  door and says, "Ticket, please."
 

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