An old chestnut
goes something like this:
In a bar near the border between Whozitland and Whatzitland, a man in Whozitland pays for a 10 cent beer with a Whozitland dollar, and receives a Whatzitland dollar (worth 90 cents in Whozitland) in change.
He drinks the beer, then crosses the border into Whatzitland, goes into a bar there, and buys a 10 cent beer, pays with the Whatzitland dollar he just got in change from the other bar, and receives a Whozitland dollar (worth 90 cents in Whatzitland) in change.
He repeats this process, each time buying a beer in one country with their dollar and receiving the other country's dollar in change, until he's too drunk to cross the border. At the end, he still has a Whozitland dollar in his pocket that he received in change from the last beer he bought.
Who paid for the beers?
This one really shows its age by the presence of 10 cent beers. :-o