A stranger walks into a bar, looks at the board, puts $5 down and asks for a pint. The barkeep asks if he wants the Stout or the Amber. The stranger asks for the Amber.
Another complete stranger comes in, looks at the board, puts $5 down and asks for a pint. The barkeep immediately pulls a pint of Amber.
How did the barkeep know what the second stranger wanted?
I remember hearing a variation of this back when the exchange rate between the Canadian dollar and American dollar was 10:9. Specifically $5 Canadian equalled $4.50 American.
The bar is near the Canada-American border and since the border is unguarded people from both countries often made short crossings needing only their driver's license. The price for the Stout is $5 American and the price for the Amber is $4.50 American.
The first man put down an American $5; this could buy either drink so the bartender asks which one. The second man puts down a Canadian $5; this is enough for the Amber but not the Stout so the bartender knows to serve an Amber.