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.