The instructions should have mentioned that you need a body of water.
A joke about an incompetent pair of mobsters attempting to kill the man on the roof via cement shoes; they are incompetent because they are following a step-by-step instruction manual to do this, and have overlooked the obvious: you need a body of water to throw the person with cement shoes to kill them, and now they are driving around frantically trying to find one. It's funny in part because they are not being discrete: the incompetent pair has simply put the victim on the roof for everyone to see/try to intervene.