Originally Posted by The Batlord

First you tell somebody "I got a weak back."

Then you must ask them to ask you "When'd that happen?"

You then respond "About a week back."

The real joke starts when the both of you are around other people and you repeat the "joke" to him and he reciprocates with the correct response. If done correctly all of the people around you will assume that you are both retarded. But give it time. In a few days everyone will be doing it. They'll still think it's the dumbest thing ever, but they won't be able to help themselves.