I did not write the following joke:
Six engineers and six mathematicians are attending a conference and are traveling by train.
One by one, each of the engineers goes up to the ticket counter and buys a ticket to the conference. But only one of the mathematicians does. The engineers look puzzled and one of the mathematicians says, "Optimization."
The twelve get on the same car and one mathematician stands at each end of the car. Now the engineers are really puzzled. After a while, the mathematician at one end, yells, "Conductor!" On that cue, all the mathematicians pile into the rest room and lock the door.
The conductor enters the car and announces, "Tickets, please. Tickets!" He passes the engineers and punches each of their tickets. At the end of the car, he notices the restroom is occupied and knocks on the door, "Ticket, please."
The ticket slides out from under the door, he punches it and slides it back, then leaves the car and continues to the next car.
The engineers look at each other and decide how clever the mathematicians have been, and then wink at each other.
They all attend the conference and have a good time. Upon arriving at the train station, one engineer buys a ticket and they giggle at each other. The mathematicians do not buy any. This time again, the engineers look puzzled, and the same mathematician says, "Optimization."
This time all the mathematicians sit down and the engineers have the lookouts. One engineer, peers down a couple of cars and shouts, "Conductor!" Immediately all the engineers pile into the rest room, while the mathematicians just sit there. Once the engineers are in the rest room, one of the mathematicians knocks on the door and says, "Ticket, please." The ticket slides out under the door, the mathematician grabs it and along with the other mathematicians, runs to the other rest room and they lock themselves in.
"In the real world, this would be a problem. But in mathematics, we can just define a place where this problem doesn't exist. So we'll go ahead and do that now..."