Comment by Klathmon

I think the answer is that water has polarity! Though we’re definitely hitting my chemistry knowledge limit...

While water’s overall charge is neutral, its charge isn’t distributed symmetrically ans tbis makes it a rather polar molecule. This means part of molecule have different charges which leads to attraction to other water molecules thanks to van der waals forces. Basically water naturally clumps up and won’t squeeze through tiny holes even if it’s kinetic diameter would technically allow it. (Though I believe very high pressure can force it through)

Water’s polarity is what causes surface tension and droplets! For comparison, gasoline is relatively nonpolar, though not as nonpolar as He. If you’ve ever spilled some you can see it spread out immediately and avoid the droplet formation and cohesion you’d see with water. Similarly, water tight containers might not be gasoline tight. (but don’t test this out...)

To correct my comment from earlier: Helium is special because of its small kinetic diameter AND because it’s naturally found in a monoatomic and nonpolar form, as are all noble gases. This lack of polarity leads to minimal intermolecular attraction that would cause clumping and limit flow.

(I found a reference that helium can be measured flowing through a 1.0nm hole, albeit at a very low rate. I wonder how close to the 0.260nm kinetic diameter a hole can get for He to still flow)