The Two Thing Game devised by Glen Whitman is a hoot.
It began when Whitman was chatting with a stranger in a bar a few years ago.
When he told him that he was an economist, the stranger said, "Ah. So . . . what are the Two Things about economics?”
The eventual reply: "One: Incentives matter. Two: There's no such thing as a free lunch."
Some other Two Things things here.
Hat tip to Sam de Brito at All Men Are Liars