$\textbf{Who Paid for the Beer}$
Joe lives in a town along the US-Canada border. One day, both countries' currencies are discounted $10\%$ on the other side of the border. That is, a US dollar is worth $90$ Canadian cents in Canada, and a Canadian dollar is worth $90$ US cents in the US. Joe buys one US dollar's worth of beer in the US. He pays using a ten US dollar bill and receives ten Canadian dollars as exchange. Then, he walks across the border and buys one Canadian dollar's worth of beer. He pays using the ten Canadian dollars he has and receives ten US dollars as exchange. He then walks back to the US side to buy another US dollar's worth of beer. He receives ten Canadian dollars as the exchange before goes to Canada again. After coming back and forth, Joe finally returns to his home and becomes dead drunk. However, he still has ten US dollars in his hand. The question is who has paid for all the beers Joe has consumed?
$\textbf{Answer}$
There can be different answers to this question. One of them is Joe pays for all these beer himself by his work.
$\textbf{Note}$
This is an economic problem. Effectively, Joe gets all these beer by arbitraging the value of the currencies in different countries. He makes this possible by his work, i.e. traveling across the border and trading two currencies. He essentially sells ten US dollars for a beer and ten Canadian dollars in the US side where US dollar is valued relatively higher. Then, he transports the ten Canadian dollars to Canada where Canadian dollar is valued higher and sells them for ten US dollars and a beer.
From economic perspective, this is an excellent example of the fundamental benefits of trading.