It’s one of those everyday mysteries that sits right in front of us — literally in our fridge — yet few ever stop to ask: What is that circular dent on the side of a plastic milk jug for?
Most people will go their entire lives seeing it, touching it, pouring milk right past it, without ever realizing that it serves a very real and very clever purpose. It’s not a design flaw, a random indentation, or a place to grip (though that’s a common guess). That dent — sometimes called a “push-in” or “expansion dimple” — is a small detail hiding a big job.
Let’s uncover what this unassuming little feature is really doing.
First, What Is It?
Look at a standard one-gallon plastic milk jug, and you’ll notice a circular or oval-shaped indent on one or both sides. It’s recessed into the plastic, usually about the size of a tennis ball. Unlike the handle or spout, this feature doesn’t seem obviously functional — but it is.
The Real Reason Behind the Dent: Pressure Control
That dent is designed to absorb changes in pressure inside the jug. Here’s how:
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