Neoclassical Economics

A physicist, a chemist and an economist are shipwrecked on a desert isle, along with a container full of cans of baked beans.

The chemist says that if they can start a fire, he can calculate the temperature at which a can will explode.

The physicist says that she can work out the trajectory of the baked beans after the explosion, so that they can gather the baked beans and eat them.

The economist looks at them in disdain, and finally says "Guys, you're going about it the hard way. Let's assume we have a can opener".