I want to eat this cake, but I'm already overweight. But no problem, the baker tells me. As it happens, he was planning to put out another cake with the same a

I want to eat this cake, but I'm already overweight. But no problem, the baker tells me. As it happens, he was planning to put out another cake with the same amount of calories tomorrow, which I'd surely eat too. But if I pay him a dollar, he'll hold off doing that and hand me a fancy certificate documenting his intention. I can then take the negative calories that spring from my act of not-eating the hypothetical future cake and use them to offset the positive calories of the actual cake right in front of me - and in the end, the cake will have net zero calories and I can finally eat in piece. Yum!