You’ve probably held a plastic milk jug hundreds of times without giving its design a second thought.

But have you ever noticed that strange circular indentation on the side?

It looks almost like someone pressed a large thumb into the plastic, leaving behind a perfectly round dent.

Most people assume it’s simply part of the manufacturing process or a decorative feature.

Others wonder whether the container was accidentally damaged before reaching the grocery store.

But that little circle isn’t there by accident.

In fact, it is a carefully designed feature that helps the container handle some of the challenges of everyday use.

From the moment a jug is filled at the dairy to the time it reaches your refrigerator, it experiences temperature changes, movement, and pressure.

And that unusual indentation can help the plastic respond to those conditions.

But the most interesting part is what can happen to that circular dent when pressure inside the jug changes.

Continue below in Part 2