Milk in bags — it's a kitchen staple in some parts of Canada.

Yet there are areas of the country where the concept of four litres of moo juice coming in transparent, jiggly bags is the most inexplicable thing about life in Eastern Canada.

But what's to blame for the proliferation of bagged milk in Ontario, Quebec, and Maritime provinces but not others?

It's a question The Cost of Living heard from listeners who were confused when they had difficulty finding the same milk jugs in Ontario that are easily sourced at any grocery store from British Columbia to Manitoba, or in the United States.

This doesn't make any sense. Why would they do it like this? - Shemma Yamach, puzzled by milk bags after moving to Toronto from Edmonton

"Whenever I have American friends visiting, they think the milk in my fridge is totally ridiculous," said CBC reporter Jonathan Pinto. "In Detroit, milk comes in jugs."

Even chocolate milk comes in bags in parts of Canada. (Canadian Food Inspection Agency)

The reaction is similar for people who come eastward from western provinces.

"Coming from Alberta, I'm used to seeing milk jugs. I went through the store repeatedly and couldn't find them," said Shemma Yamach, who moved to Toronto after growing up in Edmonton.

She quickly realized the issue wasn't that every grocery store she visited was just out of what she thought was universal — the four-litre hard plastic milk jug.

Those who use milk bags also know to have a reusable plastic holster for said bags. (City of Toronto)

"After going to people's homes and seeing that everyone had these bags of milk, and they cut them open on the corner and drink milk out of this pitcher… well, I just thought this doesn't make any sense. Why would they do it like this?" asked Yamach.

The question was echoed by Pinto, who works for CBC Radio's Afternoon Drive in southwestern Ontario.

"How come some places have milk in bags, and others don't?"

Enter the metric system

Milk bags first entered the Canadian market in the late 1960s.

However, Canada's conversion to the metric system in the 1970s meant dairy producers needed to replace and resize existing milk containers, which were measured in imperial quarts.

When buying milk in plastic bags, consumers have to buy a jug to hold it too. The practice goes as far back as 1969. (CBC News/CBC Archives)

Retrofitting assembly lines or replacing heavy glass bottles was an expensive prospect for the milk industry, and milk bags — which they were already experimenting with — could be easily and cheaply adjusted.

Changing a one-quart bag to a 1.3-litre bag was relatively painless, so three-quart bags of milk quickly became four-litre bags across parts of Canada.

Where my bags at?

Milk bags started to fall out of popularity in many parts of the country as the hard plastic used for things like large jugs became cheaper.

Plus, in the 1980s, Brian Mulroney's Progressive Conservative government relaxed rules on metric measurements.

This combination helped market forces take hold, and milk jugs slowly became more popular in areas of the country.

Milk, without bags, is pictured at a B.C. grocery store in Sept. 2018. (Jonathan Hayward/The Canadian Press)

"I think the shift took place less as a result of a movement away from plastic bags, and more the attraction of rigid milk jugs," said Dan Wong, president of the Western Dairy Council.

Eventually milk bags became unheard of in provinces like Alberta or British Columbia.

Why the bag for Ontario?

Ontario has remained an exception.

For decades, regulations in Ontario restricted the sale of more than one pint or about 473 millilitres of milk in containers other than plastic film pouches (aka bags), laminated containers or coated paper containers (such as Tetra Paks).

"I think it was a historical regulation that stemmed back to the days when plastic jugs were very rare in the marketplace," said Dan Wong.

To sell milk in four-litre hard plastic jugs, a retailer or producer had to implement a deposit or recycling system for those products and some stores, such as the Becker Milk Company, did so. Consumers could buy milk jugs at those retailers if they paid a deposit for the jug at the store.

Stunned by the size of these giant milk bags sold in supermarkets in <a href="https://twitter.com/hashtag/Canada?src=hash&ref_src=twsrc%5Etfw">#Canada</a>! <a href="https://twitter.com/hashtag/milk?src=hash&ref_src=twsrc%5Etfw">#milk</a> <a href="https://twitter.com/hashtag/travel?src=hash&ref_src=twsrc%5Etfw">#travel</a> <a href="https://t.co/HDVKVDicqo">pic.twitter.com/HDVKVDicqo</a> —@carolchan9394

Bags did not have this restriction, so mainstream grocery stores and milk producers stuck to the bags for the most part. This explains why Ontario grocers almost exclusively provided large quantities of milk in bags.

In other markets such as Quebec or Nova Scotia, bags and jugs have co-existed for years based on demand.

The Ontario regulation was amended in mid-2018, but consumer habits can take decades to break, so expect to keep seeing white, jiggly milk bags at your local grocery store for years to come. At least in Ontario.

Written and produced by Anis Heydari.

Click "listen" above to hear the segment, or download the Cost of Living podcast.