We all know that glass is really fragile.

And when it breaks, it usually explodes  into dangerous shards that go everywhere. Which is less than ideal  when that glass is used for, like, car windows or the outsides of buildings.

And how can glass be bulletproof if  most glass isn’t even floor-proof? Well, over five thousand years of us making  the stuff, we’ve developed some tricks for making glass stronger when it’s  whole, and safer when it breaks. And we’ve also decided that you can call something  “glass” when it’s not really, well, glass.

Or, at least, not just glass. [♪ INTRO] Now, glass is a solid, and no matter what  keyboard warriors say, it is NOT a liquid. That said, it’s what we call in the  world of science, a weird solid. Okay, technically the word  is “amorphous,” but still.

See, most substances form  crystals, where atoms and molecules get locked into a pattern as  the liquid freezes into a solid. But not glass. There’s no one most natural way  for its molecules to lock together.

Some areas end up with molecules arranged one way and some areas are completely different. The transition from liquid to  glass is so strange that no one fully understands it: It’s an  unsolved problem in physics. And the end result is a chaotic mess of  molecules that looks more like a liquid than a typical solid crystal shape,  hence the term amorphous solid.

Those messy molecules help make glass transparent; the molecules are so disorganized that  they don’t absorb any one visible color, meaning that all the colors of visible  light can pass through, more or less. But glass’s disorganized  molecules also make it brittle. The molecules end up in tightly  packed clumps in some places, with lots of force binding the  clumps together internally, but a lot less force holding  the separate clumps together.

And those areas with less force are  all throughout the piece of glass, meaning it’s chock full of weak points. If the glass is hit or bends  along one of those weak points, the clusters might just separate permanently,  forming a tiny crack in the glass. And those cracks will grow and  keep driving clusters apart, until one last impact makes  the whole thing go kaboom.

Those cracks spread across all  dimensions of the piece of glass, too, creating super-sharp edges that can  be as thin as a single molecule. Once a growing crack reaches the  edge or circles back on itself, energy stored between molecules is  suddenly released and the glass shatters. That makes glass sound like an  awful choice for, say, a car window.

You know, something that’s constantly bonked by little stones and sometimes  hit by an entire car. Fortunately, cars use a  specially designed type of glass that’s resistant to shattering - at  least, shattering in a dangerous way. Car windows usually have  heat-treated or tempered glass.

Manufacturers melt sand and other stuff  like they normally would to make glass, but they don’t let it cool  down like they normally would, with basically all parts  solidifying simultaneously. Instead, they force the  outermost layer to freeze first. The liquid center would usually shrink as  it freezes, kinda like an under baked cake.

But it can’t do that, because  the edge is already solid, so there’s no slack for it to sink down with. This creates a solid pane that’s  constantly pulling against itself, which also makes it resist anything  external that might pull it apart. Tempered glass can be up to five  times stronger than untempered glass.

And when it does break, all that  tension gets released at once, immediately creating countless  weak points in the glass. Cracks immediately find other  cracks, so they can’t create those molecularly sharp edges that we  usually worry about with broken glass. Releasing all that tension at  once means that tempered glass doesn’t really shatter into big pieces.

It explodes. But it’s an explosion of dull-edged clumps, so the pieces are much safer to be around. Windshields go one better than tempering  and put a thin sheet of plastic between two panes of tempered glass  to make what’s called laminated glass.

The sticky plastic holds the glass in place  even if it breaks a little bit, and anything that might go all the way through the  windshield first has to travel through a strong, tempered pane; then sticky, bendy  plastic; and then another tempered pane. Because it holds together and has all  those layers, laminated glass is even stronger than the same thickness of  tempered glass would be on its own. And bulletproof glass takes  this idea to the next level.

If laminated glass is a tempered  glass sandwich, bulletproof glass is a laminated glass sandwich: Glass, plastic,  glass, plastic, glass, plastic, glass… And so on until it’s strong enough to stop  a bullet, while still being see-through. That said, glass is heavy, especially  once you start making sandwiches with it. In most true bulletproof glass,  some or even all of the glass is replaced with a lighter plastic  like polycarbonate or acrylic, or designers might just go for one really  thick sheet of one of those plastics.

So really, we might be better  off calling bulletproof glass “bulletproof-glass-and-plastic-and-other-plastic-and-glass  sandwiches that sometimes don’t have any glass at all”. But that doesn’t quite roll off the tongue. So there’s a lot more to making resilient, sturdy, crack-proof glass than meets the eye!

Now, if only researchers  would apply all this knowledge to create a true shatterproof wine  glass, then we’d be in business… Thanks for watching this episode of SciShow. We want to give an extra shoutout  to our patrons on Patreon, who help us keep the lights on  and the science stories flowing. Patrons also get some neat perks,  like our patron-only podcast, a whole backlog of blooper reels, and  even a patron-only Discord server.

If any of that sounds like it’s up your alley, head over to Patreon.com/scishow to check it out. [♪ OUTRO]