This one you can rationalize as the moneyless/barter exchange of "I will eat a shit if you eat a shit" or if you see as an "arbitrage", "we both value eating a shit slightly lower than making the other eat a shit" thus we can make the other eat a shit for the very low price of eating a shit ourself.

Or you can see it as a very basic representation of money flowing in the opposite way of services and goods in the economy, you can even measure the speed of money with the amount of shit they make each other eat in an afternoon.