Why does water gurgle when it’s flowing out from a bottle?

We know that air has pressure, and so, if there is an empty space anywhere, the air will press into it. Now, when we pour water out of a bottle which is full, there must be an empty space left behind in the bottle when the liquid comes out, and from moment to moment, as that empty space forms in the bottle, the air outside is bound to rush in to take its place. If the bottle has a wide mouth like a tumbler then as we pour the liquid out, air can flow in evenly and there is no gurgling.

But if we take a full mineral water bottle and hold it upside down, then there is a series of fights going on between the liquid, which is trying to get out under the pull of gravitation, and the air, which is trying to push its way past the liquid to fill up the space in the bottle. Sometimes the air pushes back the water, and sometimes the water pushes back the air. This means that the air is thrown into little disturbances, which we hear as gurgles. We say that water gurgles, but it is the air that is disturbed by this contest between it and the water. We call these disturbances ‘gurgles’.

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