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So today I was in basketball practice and a friend of me showed me his regular plastic water bottle. When he squeezed it at the middle of it a small stream of water started flowing out from the bottom of it. When he stopped squeezing the flow of water out of the bottle stopped. I was annoyed as I wasn’t really able to explain why the water doesn’t flow out of the bottle if you don’t squeeze it. Why does this happen?

Thank you in advance!

[I did this experiment in my home and here are two photos: 1. I don’t squeeze the bottle and 2. I squeeze the bottle and water flows out] (https://i.stack.imgur.com/maNXR.jpg)enter image description here 2nd photo here (https://i.stack.imgur.com/Czrcx.jpg)

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Water will leak out of the hole until the pressure just inside it equalizes with that just outside (it will actually stop a bit before that due to surface tension, which can hold back a small pressure difference when the water surface bulges out through the hole). If there is, say, a 10 cm column of water above the hole, that contributes an excess pressure over atmospheric of 1 kPa, which will push water out until the air space above the water expands by about 1%, at which point the decrease in air pressure cancels the hydrostatic pressure from the water.

It the hole were much bigger, air would be able to get in there, and, glug glug, the water would all come out. But because of surface tension, you can’t simultaneously get water to go one way and air the other through a small hole.

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  • $\begingroup$ Thank you for your answer. Also figured out that by removing the cap water starts again leaking out of the bottle as air pressure is also there on top of the bottle then (?) $\endgroup$ Feb 16, 2021 at 20:23

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