(KTXL) – Just about every plastic milk jug has a circular indent on its side, but why? Is it just decoration? Does it help determine expiration?

Some people may not have ever noticed it. “Most people will go their whole lives without ever knowing what the circular indent on the side of the milk jug is for,” one recent post shared to social media theorized, prompting hundreds of responses.

“When it pops out, the milk is going bad,” one Facebook users named Cory Andersonclaimed.

Another social media user suggested the indent has to do with the carton’s ability to expand in cold temperatures.

“It’s so if you freeze the milk the bubble gives it someplace to expand to so it won’t bust the carton,” said a Facebook user named Andrew Serpico.

According to The Dairy Alliance, an organization that promotes dairy foods and knowledge about the dairy industry, the indent, also called a mould, isn’t just a stylistic decision. It helps control the amount of milk filled into the jug, and strengthens the container enough to hold the milk inside.

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

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

The circular indent, too, plays a role in protecting milk as temperatures change. During the summer, plastic shrinks due to the heat, so the indents are made slightly larger to prevent milk overflow, according to Food Republic, a news outlet for recipes and cooking tips. In colder months, the indents are smaller, but consumers reportedly get the same amount of milk all year long.

Product development technologists Lauren Joshi and John Cowan also wrote in a June 2017 report that bottle shrinkage is nearly unavoidable in the milk-producing industry, as containers made from high density polyethylene (HDPE) shrink after coming off the manufacturing line. The indent, therefore, help control production costs by preventing spilled milk and supporting fuller-looking bottles, which are essential to consumer satisfaction.