..glass
Seriously we can see through it, we can break it with our fist, a rock or most available objects however it seems to create an impenetrable barrier through which we cannot pass.
We have glass in our houses, offices and cars and this seems to be enough to keep people out, most of the time.
Glass is fragile and prone to break with ease yet it stops us in our tracks (literally sometimes) and deters us from crossing the boundary it creates.
The glass on your mask stops the water and other critters from getting in your eyes, remember to keep it clean and use a good defogger (Johnsons Baby Shampoo works for me).
Its safer when you see where you are going.
Lets remember it's time to 'Dive For Your Life'
Good luck and don't FOG it up