is a mind challenging quiz for you. Watch this Youtube-clip
(more specifically the actions between 2:35 to 4:40 into the clip), and then ask yourself, first of all: what's the catch (after all, Beavis and Butthead are careful to always exchange actual dollars for the candy bars they buy and eat), and secondly what connection this has to Keynesian economics. I'll give you two days (until Tuesday evening) to think about it, and then I'll give you the answer to both questions.
UPDATE: The clip has now been removed. See a summary in my follow-up