Thought:
Why can you receive a Canadian quarter from an American vending machine but you can't put one in? The Coke machine nearby keeps giving me Canadian quarters in change, but it spits them out immediately if I try to use them. How did the things get in there in the first place?!?!!
That makes my neurons zap: you can spend American money in Canada, but you can't spend Canadian money in America. Tim Horton's graciously accepted my Andrew Jacksons and Abe Lincolns but, across the border, McDonald's wanted nothing to do with my Polar Bears and Loonies. Who makes these bizarre rules?