My son just asked me at dinner and I do not have an exact answer.
In: 7
It’s because the straw is narrow enough that water’s inherent “thickness” (I assume a child doesn’t know what viscosity means) stops it from just sliding out if it can’t be replaced by air. If you block one end of the straw, an air bubble cannot travel up through the water, and cannot enter through the top. So it just sits there.
But it’s important that it’s a straw. If you start increasing the diamater of the tube the water is in (to, say, bottle neck size), at some point the water isn’t “thick” (viscous) enough to overcome gravity anymore, and will buckle on one side, allowing a bubble of air to rise and replace it on the other. That’s why if you tip a bottle upside down, it doesn’t just all flow out at once either, but gurgles. It still has enough viscosity to kind of stick together. But it will flow of you spin it, because then the air enters through the centre of the whirlpool.
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