10 MDs and 10 PhDs are going to a meeting by train. The 10 MDs each have their own ticket, but the 10 PhDs (who have little money, of course) have 1 ticket between them. The MDs ask the PhDs (in a caring manner), "How are you going to manage with only one ticket?" "Just watch." reply the PhDs.

They all get on the train and the 10 MDs take their seats and hand their tickets to the conductor. But the PhDs all pile into a bathroom, and when the conductor comes by, a single arm reaches out and gives him the ticket.

The MDs, feeling enlightened, decide to try the same thing on the way home, so they purchase just one ticket between ten of them. Now the PhDs buy no ticket at all. "How are you going to get home?" ask the MDs. "Just watch." the PhDs reply.

When they get on the train, all the MDs pile into a bathroom. 9 of PhDs get into another bathroom. The tenth PhD then knocks on the MDs' bathroom door and says "Ticket please." Out comes a single arm to hand over the ticket.

The moral of the story:

DON'T USE A TECHNIQUE UNLESS YOU THOROUGHLY UNDERSTAND THE PRINCIPLE.