The double thank-you of capitalism is noticing that, when a seller and a customer give money to one another for goods, each thanks the other, showing that the transaction is not only optional, but good for everyone who was involved. This can be shown as different to the thought of a "fixed slice pie" that takes place in many other situations, with one side taking and the other giving, a thank-you would only pass only one way. The double thank-you is used in economic discussions to show an example of the first benefit of a free market, that where all actions are optional, any transactions must benefit both sides, and this helps everyone in the general community.