My son just asked me at dinner and I do not have an exact answer.
In: 7
To answer the question of why the water doesn’t fall out (which is what I think you are actually asking): Just like water is a kind of stuff, air is stuff too. It’s actually pretty difficult to get rid of all the stuff in an area, so if you don’t put your finger over the straw when you lift it out of the water, the water will run out of it, but air will rush into the other end to fill the space it left behind. If you use your finger to stop the air rushing in, the water can’t leave because it would leave nothing behind, which takes a lot of work, so instead the water stays in the straw until you lift your finger and let the air in.
To answer why it doesn’t work if you have something larger than a straw (which is related but not what I think you are asking): So if air has to replace the space the water takes, it has to either enter the straw from the end your finger is on, or it has to go up through the water. If it goes up through the water your finger won’t stop it from falling out. Water likes to stick together though, so it’s hard for the air to make it through. But if the opening is narrow enough, the water can’t hold together as well, and some water can fall out and air can make its way in.
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