My Challenge in Econ 101 explanation of Marginal Benefit = Marginal Cost In Econ 101 textbooks, there are lots of examples and emphasis on marginal analysis leading to the greatest equation of all, $MB=MC$.

My challenge is the following and wonder if anyone had a similar issue or a way to get over it:

One of the textbooks examples use consuming pizza or water. First unit brings you the biggest joy. Then it diminishes. So student would understand each additional unit of pizza wouldn't bring the same amount (e.g. initial v. when you are full) utility. But then there comes a time where you are quite full and feel satisfied. At this point, additional unit of consumption would probably make you feel sick or vomit, so it is not "worth the money" to spend on.


  The trouble with this example is that it illustrates at margin, benefit is not constant. There comes a time when you would stop consuming. But this doesn't really shed light on why $MB=MC$? How can I do a better job explaining that this "equality" is embedded in all these stories? Is it perhaps the stuffed/eat-until-vomit is a corner solution example?


For pedagogical reason, if you are better at explaining these concepts, please share how you would go about this with an example both interor and corner solution in ECON 101 textbook situation.

My strategy was from example to the graph and to show why the equality should hold leading to connecting the dots on the first order conditions of utility maximization... But this equality is hard to really spell it out on 101 level, that's my challenge.

Thanks!
So, you assume a finite stock of money units and also that the only thing any consumer desires is units of happiness. That is, consumers are utility maximizers and they have utilities that are functions of happiness only.

Now, if you impose some arbitrary value - say, one unit of money - for one unit of happiness you could construct any silly example. Say, a frat guy loves beautiful women. He can win the affection of a beautiful woman by taking her on a date that costs 10 units of money. The first day late with a beautiful girl gives him 20 units of happiness. Clearly, the date is worth it! This is beacause he is getting 2 units of happiness per unit of money which has, more or less, doubled the value of his money. Suppose he keeps dating beautiful women and that the 5th date - that costs 10 units - gives him exactly ten units of happiness. Now, suppose he knows date 6 will only give 8 units of happiness. The marginal benefit of the date is too low. He keeps his money and instead trades his units of money 1-1 for units of happiness until he depletes his remaining stock of money. 

. 

Something like that. Any arbitrary example where you can clearly demonstrate that if $MB&lt;MC$, you're "losing" something by continuing to consume that good and $MB&gt;MC$ means you're "gaining" something by continuing to consume that good. 

Also - I have no graphing tools (typing from phone) but this example makes use of constant MC and so gives a good visual for what happens when MB is above and/or below MC by presenting two very easily identified regions where the MC curve partitions the graph. Above the curve is "good" and below is "bad". 

This usually works well for me.