Good one! The key to get across is the impact that Socialism (and its variants) have on economic incentives. I like the story about Thoreau and some of the socialists societies he was involved with. All day, Thoreau sits under a tree writing poetry while a counterpart is out plowing the field, sweating his brains out while staring at the East end of a West-bound horse. At the end of the day, they retire to the lodge for dinner...to share equally in the efforts of the day. How long does it take the field hand to get PO’ed at Thoreau? Indeed, it is precisely because of the disincentives that such societies (New Harmony, Owenism, etc.) create that causes them to ultimately break down and fail.
The father then asked her if she would be willing to have grades earned that same way. Did she think that the students who didn't study and flunked tests should get an A also. Or would she be willing to give up some of her grade point average and get a C or a B so that the D and E students got C's and B's also.
She wasn't so willing to do that because she studied hard and gave up social events to get good grades and he said, well that's exactly what you are expecting good workers to do with their hard earned money.
Then she saw the light.