When drinking from an inverted bottle, a vacuum starts to appear at the top as water flows out of the bottle. Because of the air pressure outside, air forces its way through the neck of the bottle and bubbles up. This is followed by more water escaping, and more air bubbles moving up. So on and so forth. The glug-glug is caused by these two alternating processes.

Water flows faster at the beginning because this is when the pressure is highest.

The experiment can be carried out quite easily in a kitchen or bathroom. The result accords with our experience.

Water flows faster when the bottle is tilted, i.e. at an angle. This avoids the gurgling, i.e. air bubbles coming up through the liquid, which obstructs the passage of water.

The fastest way is to pour water at an angle and with a swirl. To create a swirl, move the bottle in small circles before pouring. This way, water moves to the side of the bottle and no gurgling occurs, allowing air to freely enter through the centre.