[ELI5] why does plugging one end of a straw not let the water out of the other end when held up?
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The average air pressure at sea level on earth is 1 atmosphere/atm. This pressure is constantly pushing on all things from all sides.
When you have a straw filled with water, the water falling out the bottom is constantly replaced by air coming in from the top due to this air pressure. If you plug the top, air cannot replace the falling water from the top anymore. The only way for air to get into the straw now is from the bottom… where the water is supposed to be coming out of. As air is more compressible than water, the weight of the water increases the volume of air inside the straw without increasing the mass/amount of air (like a plunger or syringe), decreasing the pressure inside the straw. As gasses want to fill spaces and the outside air pressure is 1 atm, air is pushing back up the straw, creating an equilibrium where the water cannot fall and the air cannot fill the straw.
Other things to note: this is really only possible with straws and other similarly-narrow tubes due to water cohesion (sticking to itself strongly). It’s hard to replicate this even with a 2-liter bottle, as the force of water cohesion is not enough to overcome the ambient air pressure from breaking the tension and forcing itself up into the bottle as bubbles. This is why tipping over a 2-liter causes liquid to come out slowly in bubbling bursts. Also, this couldn’t happen in space. With 0 air pressure, there’s no force pushing the water back up the straw, so it would just fall out.
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