In my experience, Canadian shopkeepers don’t really care if you give them a US coin or two, but Canadian coins are refused in the US, even very low denominations. Is this a legal or cultural difference?
For a long time Canadian money was worth less than American money. Giving an American quarter to a Canadian would have been slightly over paying, doing the reverse would be slightly underpaying. My experience on both sides of the border is that a store that gets a large amount of its business from tourist money is willing to go the extra mile, and a store that rarely sees a tourist probably won’t help you out.
I live right near the US-Canada border. You’re right that a lot of it is cultural, but attitudes are starting to change. More and more businesses on the US side are accepting Canadian money at par; the big outlet mall here, for example, will always accept Canadian bills or coins.
There’s still a lot of residual distaste about Canadian coins here, though. When I go to the bank to turn in wrapped (US) coins, they run a magnet over the coins to see if there’s even one Canadian coin in there. If there is one, you have to unwrap them and dig through 'em to find that one Canadian penny or whatever.
Back when I ran a Dollar General down here in GA, we didn’t have a problem accepting Canadian coins, and that was back when US money was worth more. Never had anyone try to use a Canadian bill, but we would’ve probably taken it.
Down here in Schenectady, Canadian coins were accepted equally with US coins. We’d just give them back in change; no one objected.
I’ve been on both sides of the border and have received at least one coin from the other country. It isn’t a big deal if it is just one dime or something.
I found that Canadian stores in tourist areas generally accepted US dollars (though, you might get a better rate at a bank or an ATM).
It’s often not the store’s decision.
I own a store in Montana, which shares a long border with Canada. We get a lot of Canadians here, and I’ve never had a problem with a big of Canadian coinage. A couple of years ago, though, we received a memo from our bank saying they would no longer accept any Canadian money of any denomination.
Why is each US and Canadian coin approximately the same diameter and colour as its foreign counterpart, if it has one?
I refused a Canadian coin once and had a lady get very irate with me. Her insistence was, “It’s money!” My position was, “Not in this country. We don’t accept pesos, either.”
I completely understand places closer to Canada accepting them for good relations. Charlotte is not what I would describe as close to Canada. 
How many coins are in a big?
A metric gross.
Where I work (US), the candy machine recently gave me a Canadian nickel (5 cent piece) in change, but then refused to accept it when I tried to use it to buy something. However, the soda machine gladly accepted it.
Argh! BIT. A BIT of Canadian coinage.
Now I feel like a Looney. 
Better a Looney than a Looney Tooney
(Or is that Twony?)
Back when the northern Peso was worth about 70 real cents, I found that the vending machines on the NYState Thruway did not accept Canadian quarters and had stickers to say so. (I lost one trying it…).
Since Canada is 1/10 the size of the US, the US dollar until recently was worth a lot more, most places are within drivng distance of the border, and many people go south to shop, Canadians have no problem accepting smaller denominations of US currency. Several places even have a specific key on the computer till pre-programmed with the exchange rate to acept bills. Coins exchange at par, since the difference on a quarter at the worst time would have been less than a dime.
Generally, it’s not as easy to pass Canadian coins in the USA. Why should it be? They’re on use unless you go to Canada, especially if the local bank and some customers don’t want them, and the vending machines might not take them. The coins are sufficiently different that it is possible to program machines to not accept them, it seems.
Of course, we occasionally see weirder more foreign coins that pass without a close look. Caveat emptor.
Have you looked at a map recently?
I think md2000 meant in terms of population (Canada’s population being approximately 1/10 of the US population), although this could’ve been explicitly spelled out.
What’s the latest? I just found two looneys and a twoney. Where can I use them or convert them? A US Walmart maybe…are there WalMart’s in Canada?
I live in Maine, and most places take Canadian coins (25¢ and lower) at par. Most places wouldn’t even bat an eyelid at being handed a Canadian dime, nickel, penny, or quarter, and likely wouldn’t even notice. They’d just throw it in the till, and the next person that needed change might get one in return.
It was that way even when Canadian dollars were only worth about 80% of a US dollar. Higher denominations, and they’re going to either apply an exchange rate (which might not be as favorable as the market rate) or simply not take it.
Just keep them as collectors items. When we overthrow the US they’ll be worth more.
I think there’s a Walmart in Toronto, and maybe one in Vancouver. Not sure.