The Hidden Purpose Behind That Little Dent on Your Milk Jug

That small, round dent on the side of a plastic milk jug?
It’s not a random flaw — it’s a smart piece of design with multiple hidden benefits.

1️⃣ A Built-In Pressure Valve

Most milk jugs are made from high-density polyethylene (HDPE) — lightweight and cost-effective.
Because the plastic walls are thin, they’re vulnerable to pressure changes.

The dent works like a mini expansion valve:

• Flexes when the jug expands in heat or contracts in cold

• Absorbs compression if dropped

• Prevents ruptures and leaks, just like expansion joints protect bridges

That little pop you sometimes hear? It’s the dent adjusting to pressure — doing its job.

2️⃣ A Shock Absorber

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