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Just a simple question : when we remove a stopper of an upside-down bottle, water flows out. But the pressure caused by the mass of water's column is usually smaller than atmosphere pressure unless we use 10m height bottle. How could water flow out? sorry for stupid question...

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    $\begingroup$ Because the air pressure is already acting on the top of an upside down bottle, which cancels the pressure at the bottom. Or simply water is filled at air pressure $\endgroup$ – N.S.JOHN May 22 '16 at 11:49
  • $\begingroup$ I'm sure you've heard the old saw - the only stupid question is the one you don't ask. I used to tell my students - I like stupid questions because those are the ones I can answer :) Regardless, it depends on the width of the opening. If the opening is wide enough, some water can fall out on one side while air goes in the other side, kinda like Whac-A-Mole :) $\endgroup$ – Mike Dunlavey May 22 '16 at 13:17
  • $\begingroup$ Not so stupid. It is the basis of this trick. physicscentral.com/experiment/physicsathome/magicwaterglass.cfm $\endgroup$ – mmesser314 May 22 '16 at 13:27
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If the mouth of the bottle is small (e.g., a wine cork with drilled hole for a drinking straw), water indeed won't flow out. Would you also wonder why the water doesn't stay in an upside-down bucket (the limit case for a bottle with a very wide mouth)?

What happens is that air enters through the mouth with a volume equal to the volume of water that leaves the mouth. If you start with a flat water-air interface surface that is perpendicular to the mouth, the surface has to deform. With small openings, the surface tension of the water will win from the gravitational pull on the water surface. With a big opening, gravity wins.

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  • $\begingroup$ Very well said... The key word here the surface tension... Try do the same with higher viscosity liquid such as the honey... $\endgroup$ – soosai steven May 22 '16 at 15:18

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