There is a circular indent on the side of almost all plastic milk jugs, but why? Is it merely ornamentation? Does it aid in figuring out expiration?

It might have gone unnoticed by some. According to a recent social media post that received hundreds of responses, “Most people will go their whole lives without ever knowing what the circular indent on the side of the milk jug is for.”

According to one Facebook user, Cory Anderson, “the milk is going bad when it pops out.”

According to a different social media user, the indent may be related to the carton’s capacity to expand in cold weather.

According to Andrew Serpico, a Facebook user, “the bubble gives the milk somewhere to expand to so it won’t bust the carton if you freeze it.”

The indent, also known as a mold, is more than just a fashion choice, according to The Dairy Alliance, a group that advocates for dairy products and information about the dairy industry. It makes the container sturdy enough to hold the milk inside and aids in regulating how much milk is put into the jug.

Furthermore, spoiled milk has nothing to do with the indent.

According to the Dairy Alliance, “the indent wouldn’t pop out because the milk’s age is just a little too old, even though such an invention would be helpful in a world of confusing dates on food packaging.”

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Additionally, the circular indent contributes to the protection of milk from temperature changes. According to Food Republic, a news source for recipes and cooking advice, the indents are made a little bit larger to avoid milk overflow because plastic shrinks in the summer due to the heat. Although the indents are smaller during the colder months, consumers are said to receive the same quantity of milk throughout the year.

According to a June 2017 report by product development technologists Lauren Joshi and John Cowan, bottle shrinkage is almost inevitable in the milk-producing sector because high density polyethylene (HDPE) containers shrink after being manufactured. Therefore, by avoiding milk spills and promoting fuller-looking bottles—both of which are critical to customer satisfaction—the indent helps control production costs.