Most of us don’t give a second thought to that small, round dent on the side of a plastic milk jug. At first glance, it looks like a design fluke—maybe even a manufacturing error. But that unassuming indentation? It’s a brilliant piece of intentional engineering packed with purpose.
It’s a built-in pressure valve.
Milk jugs are usually made from high-density polyethylene (HDPE)—a lightweight, affordable plastic. Because they’re designed to be thin (hello, material efficiency), they’re vulnerable to pressure changes. That dent flexes with expansion and contraction, helping prevent ruptures when the jug is bumped, dropped, or exposed to temperature shifts. It’s like an expansion joint in a bridge—quietly maintaining structural integrity behind the scenes. That pop you hear when you set it down? That’s the dent, doing its job.
It’s also a shock absorber.
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