In an effort to keep my reusable coffee cup clean, before washing it I rinse it out and half fill it with near-boiling hot water (from an instant hot water tap at work), and invert it aimed at the sink.

To my surprise, at least the first time, the hot water shot out the spout/mouth hole!

A Greggs Coffee cup with a small mouth hold and an even smaller air hole

I don't think the same thing as Why do steam and hot water spray from my thermos flask? is happening, and a less vigorous reaction happens if I cover the mouth hole and air hole and try to swirl the cup.

Why would this happen?

  • 3
    $\begingroup$ The hot water shot out the hole when you inverted it? Isn't this what you would expect? $\endgroup$ Commented Jun 22, 2022 at 17:08
  • $\begingroup$ @foolishmuse no.... I'd expect it to pour out. $\endgroup$ Commented Jun 23, 2022 at 13:45

1 Answer 1


This is a weaker form of the thermos-squirting effect. Sloshing the hot water (even gently) increases its surface area and therefore increases the evaporation rate into the air inside the cup. This raises the pressure inside the cup, which forces hot water out the hole.


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