Why do you hear a glugging sound when you pour a liquid out of a bottle by holding it upside down?

Asked by: Matt

Answer

That is just air rushing in to fill the space left above
the liquid. As the liquid leaves, the pressure above it
decreases until the larger atmospheric pressure outside
the bottle forces air in to equalize pressure.

You'll notice that if you pour out the liquid gently,
leaving an open space between it and the top of the bottle
opening, air can enter more gradually, without the
'glugging'.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'