Brow Beat

Why Do Diners Give You Leftover Milkshake in That Metal Container?

Answering The Lovebirds’ most pressing question.

Kumail Nanjiani sitting at a diner booth holding a metal milkshake cup with question marks drawn around it.
Photo illustration by Slate. Screengrab from Netflix.

In an early scene in Netflix’s new romantic comedy The Lovebirds, Issa Rae and Kumail Nanjiani’s titular couple, on the run from the law after a traumatic and deadly accident, pauses at a diner to ask one of life’s great questions: Why do diners give you leftover milkshake in those metal containers? “They can’t just measure it out?” Nanjiani’s character asks. “They don’t do that with other stuff. They’re not like, ’Here’s spaghetti and some more spaghetti on the side. We made too much for the spaghetti plate.’ ” So why are shakes served that way? We called up diner owners across the country to get to the bottom of this mystery.

The answer turned out to be more complicated than we anticipated—and involves several factors. Art Shibley, the owner of the Yours Truly diner in Hudson, Ohio, began by attributing it to Depression-era thinking and the principle of “waste not, want not.” “If you have that little bit of extra in there, don’t throw it out or wash it down the sink,” he told me. “You might as well give it to the customer.”

But, as The Lovebirds suggests, this alone doesn’t answer the question. Why couldn’t the person making the shake then simply measure out the right amount of ice cream, ice, and milk to fit the glass? “Measuring out ice cream is not all that straightforward,” Shibley explained. “For example, we put almost three scoops in each shake, but the scoop sizes aren’t all exactly the same. Some have more, some have less. And if the ice cream is soft, it will change the amount you end up with. Plus, we don’t measure the milk exactly. We just eyeball it.” When asked why they haven’t found a way to be more precise, Shibley answered, “It’s just the way we’ve always done it,” adding “who wouldn’t want a little extra milkshake?”

Gus Laggis, who has owned Bridgehampton, New York’s Candy Kitchen diner since 1981, had a different take. The milkshake left in the metal cup wasn’t “extra” at all, he said, but rather part of a single serving. “We make the milkshakes all one size with the machine and then we serve them in the metal cup with a glass on the side. If there are two people sharing, we serve two glasses.” Why not serve the concoction in a bigger glass? Laggis brushed off the question. “Why would we buy bigger glasses? Just so people don’t have to pour from the metal cup? Isn’t that part of the fun?” The point about sharing, also, is an interesting one. Perhaps one reason for the large size of standard commercial milkshake makers, many of which use stainless steel cups of 28 or 30 ounces, is that they were designed with sharing in mind. We reached out to Hamilton Beach, maker of some of the most classic milkshake mixers, for comment, but received no response.

Even setting the aside why a milkshake would be served in two containers, why would the extra be served in the metal container, specifically? After all, what other food can you think of that is served in the container it was made in? Eggs are not usually served on frying pans. One possibility is that it is more efficient than dirtying another dish, and diners, which are known and loved for their simple, inexpensive food and down-to-earth attitude, tend to prioritize efficiency over presentation. But Laggis also added that there may be another practical reason for using the metal cup: It helps to keep the milkshake cold for a little longer, he says, which keeps it from melting to soup before it arrives at your booth.

Plus, at this point, some diner customers have come to expect the extra milkshake. To them, those few additional ounces of frozen goodness feel less like a bonus than an essential part of one’s order. In a 1994 article on the history of the milkshake in the Baltimore Sun, Jacques Kelly wrote about this phenomenon: “Any milkshake served solo in a plain glass without the metal sidecar wasn’t worth paying for.”

In the end, there was no single explanation, but rather the answer involved a variety of factors—temperature, efficiency, the difficulty of measuring ice cream, and a desire to avoid waste—mixed together. One final factor may be every diner’s secret ingredient: nostalgia. The diner experience is largely predicated on America’s sentimental memories of the old-fashioned drug stores and soda shops of the middle of the 20th century, and they trade less in innovation than a comforting sense of tradition. At the end of our conversation, Shibley paused and thought for a moment. “It’s hard to answer the why,” he told me. “I’m not sure why exactly. It’s just how it’s done. It’s the way we’ve always done it.”