McDonald's uses a secret McChicken sauce with a cult following you didn't know existed

You might have assumed that McChickens around the world are topped with simple mayonnaise - but you'd be wrong. 

When McDonald's Canada announced on Tuesday that it was launching McChicken sauce, along with Big Mac and Filet-O-Fish sauces, in grocery stores, many were baffled by the enthusiastic response. 

After all, McChickens are simply topped with mayo - right? 

In fact, a closer look at the ingredients page reveals that the American McChicken's "mayonnaise dressing" isn't exactly mayonnaise, as the inclusion of thickeners like xanthan gum likely puts it outside of the FDA's strict definition of the condiment. 

And, in Canada, McChicken Sauce is a noted departure from mayo - specifically with the inclusion of mustard. 

Here are the ingredients in Canadian McChickens' sauce versus their US counterpart: 

This slight difference goes a long way in explaining why Canadians were freaking out over the news that they would soon be able to buy McChicken sauce by the bottle. 

Maybe when McChicken sauce is sold at grocery stores, Americans will be able to smuggle some past the border and figure out what exactly the US is missing out on when it comes to McChickens.