If you look at a (white, opaque) Starbucks coffee lid (and the lids of many other coffee brands as well) you will see in addition to the obvious sip hole, one or more tiny perforated holes on the top of the lid. The small holes, less than 1 mm in diameter, are formed by a needle-like object punching upward from beneath the lid.

What purpose does this tiny hole serve? My guess is to keep a stack of lids from staying stuck together, but that is only a guess.

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closed as off-topic by sammy gerbil, AccidentalFourierTransform, stafusa, Cosmas Zachos, Kyle Kanos Jun 19 '18 at 22:15

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The hole lets air come in. So when you drink, an under-pressure does not form inside, allowing the coffee to come out in a continuous fashion.

  • $\begingroup$ Well, if you cover the small hole up, it definitely has trouble pouring, even with water. I didn't think the hold would be big enough to matter, but the pour hole is small enough to close off due to surface tension of the water. Frothy coffee would be even worse. $\endgroup$ – Jiminion Jun 21 '18 at 12:44

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