Take a walk through any Ontario-area grocery store and you'll see something a little unusual: shoppers hefting an item into their cart that looks like a plastic package of diapers, weighs roughly nine pounds, and requires some minor effort to enjoy.

It’s a large, tasty bag of milk.

fw_gadge, Flickr // CC BY-SA 2.0

Actually, it’s three medium-sized bladders of milk, packaged together in one large sack. At home, the milk is placed in a pitcher and one corner of the polyethylene plastic is snipped off with scissors for pouring. (Some Canadians snip a second, smaller hole to let air out.) Because it’s not fully sealed, the milk needs to be enjoyed relatively quickly.

For dairy enthusiasts used to the convenience of a resealable container, all of this might seem unnecessary—yet at least 75 percent of all milk sold in Ontario comes in this unique delivery system. The bags of milk can also be found in Quebec and the Maritimes.

Why? Thank the metric system.

By the late 1960s, glass bottles were still being used for milk, but officials knew they were causing a considerable amount of waste and expense: The heavy bottles were a pain to transport and broke easily. A few years later, Canada was busy converting to the metric system, requiring liquids to be sold in liters. Manufacturing plants producing plastic jugs or cartons (which had debuted around 1915) found that their machines would have to be dramatically altered to allow their containers to be re-sized to meet the new requirements. But the process for injecting milk into plastic bags, which were introduced by DuPont in the late 1960s [PDF], needed only minor tweaks. The bags also produced less packaging waste, since they require less plastic to hold the same amount of milk. Suddenly, pouring milk into giant, floppy sacks seemed like the most obvious thing in the world.

Andrea Vail, Flickr // CC BY-ND 2.0

By the early 1980s, the metric system was fully adopted in Canada; in 1978, four-liter packages of milk became the norm in Ontario [PDF]. Buying bagged became habitual for shoppers, who realized that some of the perceived drawbacks were actually beneficial. Sure, the milk could lose its freshness quickly, but because the packaging was broken up into three bags, there was always a new one to open; unused bags could be stored horizontally in refrigerators in spots where a tall jug wouldn’t fit.

While the unusual packaging confuses even Canadians in other parts of the country, it’s slowly been gaining support in other parts of the world. UK-based Sainsbury’s rolled out two-pint bags around 2010, offering a free pitcher as an incentive for people to make the switch and cut down on waste. Some schools, like Golden Hills Elementary near Omaha, Nebraska, let kids sip from tiny, Capri Sun-esque milk pouches. You can also find them in South Africa, Hungary, and China, which also happens to traffic in bagged beer.

Not planning on traveling outside the country? Try hitting up a Kwik Trip or Kwik Star convenience store, where locations in Wisconsin, Minnesota, and Iowa sell bagged milk by the half-gallon. Though they caution first-timers might need to get used to the pouring technique—there’s apparently a learning curve—they promise customers “will grow to appreciate” the lactose consumption customs of other parts of the world.

This story first ran in 2016 and was updated in 2021.