Three software engineers and three hackers are taking a train to a convention.
On the trip to the convention, each of the engineers buys a ticket, but they notice that the hackers only buy one ticket for the three of them. They ask the hackers how they all plan to ride the train on only one ticket. "You'll see," they reply.
When they're all on the train, the engineers watch the hackers eagerly.
Just before the ticket collector comes by to collect the passengers' tickets, all three hackers pile into the same train bathroom. When the ticket collector passes by, he knocks on the door and says, "ticket, please." One of the hackers opens the door a crack and sticks their ticket out. When the ticket collector leaves, they pile back out and enjoy the rest of the trip.
For the return trip, the engineers decide they want to try the same trick. But this time, to their surprise, they notice that the hackers don't buy any tickets at all. The engineers ask the hackers how they plan to ride without any tickets. "You'll see," they reply.
When they're all on the train, the engineers again watch the hackers eagerly, but all they do is sit and smile. When the ticket collector is about to come by, the engineers all pack themselves in a bathroom stall, while, much to the engineers' confusion, the hackers remain seated.
After all the engineers are in the bathroom, two of the hackers rush into the other bathroom and wait for the third.
The third hacker goes to the bathroom the engineers are hiding in, knocks on the door, and says, "ticket, please."