There are two types of people in this world: those who get their milk in cartons and those who buy milk in plastic bags. And though Americans are all-in on cardboard milk cartons, or the occasional plastic jugs, throughout the rest of the world—from India and Israel to our dear neighbors Canada—milk is sold in plastic bags. America is, in fact, somewhat anomalous for selling its milk in upright containers rather than in plastic bags. So how did this happen? Why is some milk sold in bags, and other milk is sold in cartons? How do milk bags work? And, perhaps most importantly, which method of milk storage is better?

Let’s start off by making one thing clear: Milk sold in a non-resealable plastic bag isn’t substantively different in makeup or composition than milk sold in a carton or jug. The difference in form and delivery method has nothing to do with a difference in content, because whether it comes in a plastic bag or a cardboard box, it’s functionally all the same milk.

The packaging of milk, however, varies wildly—from glass bottles to the aforementioned plastic bags. In the United States, classic glass milk bottles were replaced by gable-top cardboard containers in the early 20th century because the paper containers were significantly lighter and easier to transport in large quantities. The gable-top containers are a distinctly American invention, patented in 1915 by John Van Wormer of Toledo, Ohio, though it was actually created in 1906 in California, which is part of the reason it's taken hold in the United States. Plus, the design is efficient, in no small part because there's no lid required. Simply pop out the spout and pour.

The plastic milk jug is equally simply to grasp in concept, and since using a milk carton or jug is so straightforward, the physics of milk bags are kind of baffling upon first encounter. The bags aren’t resealable, so how do you keep milk from spilling everywhere in your fridge after just one use? Won't the opening flop over and ruin everything?

It turns out there’s a whole market for milk bag accessories that’ll prevent a dairy disaster, including milk bag pitchersmilk bag organizersmilk bag holders that kind of look like milk bag pitchers but without a spout, and milk bag openers available in a variety of colors. You wouldn’t get a bag of milk without having these tools on hand—that would be ludicrous.

The way it works is that you take the bag and plop it into a milk bag pitcher or holder. Cut a triangular hole in the tip, and then pour as you would any other pitcher of a beverage. There are plenty of YouTube tutorials for the uninitiated, all of which feature some sort of milk bag jiggling.

And even though there’s more equipment involved with drinking milk from bags, it’s actually better for the environment than the alternative. In Toronto, for example, milk bags are recyclable as long as they’re rinsed out, and milk bags take up significantly less space than a jug or a carton. Milk cartons and jugs are technically recyclable, though according to the U.S. Environmental Protection Agency, over 70% of milk jugs are sent to the landfill.

Grocery stores in the United Kingdom have also been testing out milk bags instead of cartons or jugs for this very reason. Back in 2010, Sainbury’s made the switch because the milk bags would contain 75% less plastic than the jugs and ultimately “save up to 1,400,000kg of packaging every year,” according to a report from The Guardian. They also require fewer resources to produce.

So people use milk bags because they are more eco-friendly, and once you have all the accessories you need, it’s really not much more of a hassle. But even knowing all of this, there’s still something about the form factor—a jiggling bag of milk in a clear bag—that can elicit shivers, which is why Waitrose, another major U.K. supermarket brand, stopped selling milk in bags after a brief trial period. 

Besides, how else are you going to keep track of kidnapped kids if not for the sides of milk cartons?