Why does water gulp out of a water bottle with a narrow opening instead of a steady flow? For example, take a water bottle. Fill it with water and then turn it upside down.
Instead of flowing steadily downward, it gulps down in parts. Why?
 A: As water flows out of the bottle, air will flow in to replace it. If you just flip the bottle straight upside down, then because the bottleneck is so narrow, then at any given time, sometimes water will block the flow of air into the bottle, or air will block the flow of water out of the bottle. Essentially they "take turns" going in or out, which creates the "gulping" effect.
You can prevent this, if you can reserve some of the space in the bottleneck for water and some of it for air. One way to do this is to tip the bottle more slowly, so that air flows in the top of the bottleneck while water flows out the bottom. If you must hold the bottle completely upside down, you can twirl it to create a whirlpool effect (also called a vortex): in this case, air flows into the center of the bottleneck while water flows out the sides. Either way will allow air to flow in at the same time water flows out, so that everything goes more smoothly.
A: The gulping you describe is due to air being sucked into the bottle and temporarily halting the flow through the nozzle. When the bottle is filled with water, it is at a particular pressure. When you turn it over and some water leaves, the pressure is now lower in the bottle. 
Once the pressure in the bottle is lower than atmospheric pressure, air forces it's way back into the bottle. This equalizes the pressure and water flows again. Then the pressure drops, air gets sucked in, and so on. Eventually all the water is gone and the bottle is filled with air at the same pressure as the atmosphere.
A: There is an important point that the previous answers have missed. And this phenomenon is not widely known -- although water flowing out of an overturned bottle is! 
The glug-glug and the air bubbles rising up in the water are due to a fluid dynamical instability called the Rayleigh-Taylor instability (wiki). This instability occurs when a heavy fluid (water in the bottle, in this case) is suspended above a light fluid (the air outside the bottle). The heavy-over-light configuration is inherently unstable, and it only takes a small perturbation to upset the balance. 
In an overturned bottle, minuscule capillary waves at the surface (due to vibrations for e.g.) are amplified into larger amplitude waves until the point when the air in the bottom becomes a bubble, is cut off by the water, and rises inside the bottle as a "glug". 
Limiting cases of this instability are also responsible for the poor performance of fusion reactions, for the necklace shapes observed in supernova explosions, patterns found in volcanic eruptions, the mushroom shape in nuclear explosions, and the mammatus clouds formed on earth! 
If you are interested in reading more about the science behind this, read about it in the Journal of Fluid Mechanics here.
Takeaways: 


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*There is often enough air pressure to support a column of water inside a bottle. So water does not flow out of an overturned bottle merely because of the air pressure! 

*The real reason why water flows out of an overturned bottle is the presence of an instability called the Rayleigh Taylor instability that amplifies the capillary waves and causes the air to bubble through the water.

A: The "gulping" is due to the sucked-in air forming a bubble on its way up and by this briefly blocking the bottleneck. The shape of the bottle is important here. You wont see any "gulping" using a top sealed tube.
