"A physicist, a chemist and an economist are stranded on an island, with nothing to eat. A can of soup washes ashore. The physicist says, 'Let's smash the can open with a rock.' The chemist says, 'Let's build a fire and heat the can first.' The economist says, 'Let's assume that we have a can-opener...'
The reference comes from the stereotype that many economic models require unrealistic or absurd assumptions in order to obtain results. The phrase is often used by economists to describe research that has particularly unusual or unrealistic assumptions (by the standards of modern economic research)."