A SOCIAL media post has sparked a debate about why plastic milk jugs have circular indents, leading experts to reveal the real reason.

Some believed the indent’s purpose was to show whether the milk inside the jug was spoiled, but this theory was debunked.

The dairy debate started when a Facebook user shared a photo of a gallon-sized milk jug.

The post read: “Most people will go their entire lives without ever knowing what the circular dent on the side of the milk jug is for.”

One person commented, “It allows for expansion when you freeze it.”

A second user also thought it was “to provide room for expansion if frozen and pressure control.”

“It’s supposed to tell you when the milk has soured. If it’s indented it’s good if it’s pushed out it’s bad,” a third person theorized.

Another agreed with this theory, saying that the indent “pops out when it goes bad.”

But the real reason behind the indent has nothing to do with spoiled or frozen milk.

Some Facebook users were headed in the right direction with their theories.

“Think it’s something to do with storage to be delivered,” one person wrote.

Another said: “I always figured it was for balance. The one side has the handle that protrudes and the opposite side has that dent that seemed to keep the jug stable.”

The Dairy Alliance, a non-profit that promotes dairy products and knowledge about the dairy industry, explained on its website that the indent helps control the amount of milk filled into the plastic jug.

The indent also strengthens the jug to hold the milk.

Another fun fact that the non-profit mentioned had to do with the making of the jug depending on temperature.

During the warmer months, the jugs are made slightly larger in case the heat causes the plastic to shrink.

The jugs are usually made a bit smaller in colder months to not look underfilled.

“The indentations make that change easier, all while bringing consumers the same amount of milk all year long,” the Daily Alliance said.

HOW TO KEEP YOUR MILK FROM SPOILING

The Dairy Alliance also gave tips to keep your milk fresher for longer.

They said to keep your milk cold for as long as possible.

You can do this by shopping for your other groceries before grabbing a jug and heading to checkout.

Once you get home, put the milk in the back of the refrigerator as it will be colder there.

The back of the refrigerator also “isn’t as impacted as food in the front when the door is opened for those midnight snack considerations,” the Dairy Alliance said.