PEOPLE RESPOND TO INCENTIVES. The administration at a high school decides to strike a blow against obesity by removing candy vending machines? (Candy vending machines?? The powers-that-be at Milwaukee Hamilton resisted the idea of a student lounge, available for pool or ping-pong in the second half of lunch hour; there were no vending machines in the lounge; and I'm not sure anybody had the disposable income for such goodies anyway. The basketball team managed to win a state tournament despite such oppressive conditions. But I digress.)

Efficiency is the identification and exploitation of all possible gains from trade. Enterprising high schoolers recognize the gain from trade implicit in the removal of all the vending machines. A Constrained Vision details what happens next.