An anecdote...

There are three engineers and three scientists who go to a conference by train. To take the train, they need to purchase one ticket for each person. One day, the three engineers only purchase one ticket. They board the train and sit in the same compartment with the scientist. After a while, the train officer come to check for their tickets. The engineers swiftly go inside a toilet cabinet. The train officer do realize someone is hiding in the toilet cabinet. He knocks the toilet door and asks for a ticket. The engineers slip a ticket through the door. The officer checks the ticket and leaves. Thus, the engineers successfully deceive the train officer.

The scientists are amazed by the engineers' plan. Hence, on the way back home, the scientists apply the same trick. They only buy one ticket. However, the engineers are even crazier. This time, they don't even buy one ticket. The train comes and both scientists and engineers again, board in the same compartment. the train officer are coming to check for the tickets. The scientists hurriedly hide inside a toilet cabinet and wait for the officer to ask for the ticket. As expected, someone knocks the door and ask for the ticket. The scientists pass the ticket through the door and think that they have successfully deceive the train officer. However, shortly afterwards, someone knocks the toilet door again and ask for the ticket. Confused, the scientists open the door and find out that it was the officer who knocked the door just now. As they do not have the tickets, they are kicked out from the train.

What happens to the engineers then? Apparently, it was the engineers who knocked the door the first time. They took the ticket from the scientists and hid inside the other toilet cabinet. Thus, they are safe and successfully deceived the train officer for the second time. Poor scientists...