As seen on Metafilter:

An engineer, a chemist, and an economist are shipwrecked and stranded on a desert island. Luckily, several cases of catering-size cans of food have washed up on shore alongside them. Unluckily, they have no can opener. They decide to think on it for twenty-four hours, then present their solutions to the dilemma.

The engineer goes first and says “I’ve calculated the strength of the cans based on a rudimentary finite-element stress analysis, and I think if we drop a large rock onto them from 6 feet, they’ll burst open”.

The chemist goes next and says “I’ve estimated the rate at which seawater could rust through the tinplate, and I think a couple more days in the brine will do it – the cans will just fall apart”.

The economist goes last, and, looking pleased with himself, begins “Suppose we had a can opener …”