At my work we use a gizmo called a helium mass spectrometer leak detector. We use it to test the hermetic seal of a welded part. It pulls a vacuum on a part or chamber and we spray helium at it and the machine looks for helium leaking through.

The units are weird: atm-cc/sec or torr liters/sec. 1x10-6 torr/liters sec is the limit for a crack large enough to let water through. Most customers require 1x10-7 or better depending. Some really exotic stuff that goes into satellites must seal to 1x10-9 or better. So helium is really small and sneaks past "holes" water and even other gases would not penetrate.

Helium, like hydrogen are really small and can penetrate many common materials, even metal (see hydrogen embrittlement). They're also difficult to remove from a vacuum chamber and even bounce backwards through high vacuum pumps. This is actually used in the leak detector and they measure this "reverse flow" of helium through the turbo vacuum pump into the mass spectrometer. Helium don't give a fuck.

So knowing that helium is really tiny and penetrates lots of things that big fat bulky water cant means this material exhibits exotic properties that are the complete opposite of what we normally observe. That's really interesting for sure.