Welcome back to Smarter Every Day.

Have you ever been driving along

and you suddenly stop and realize that you're moving

around this world in a bubble of glass?

It's kind of weird if you think about it.

But it's really cool.

Engineers have designed the glass in your car

to be extremely safe, so that when you have a wreck

you decrease the chance that you're getting

cut in half like the bad guy in Ghost.

Remember the movie Ghost? Anyway.

So today on Smarter Every Day

I want to investigate the two different types of glass

that go into car windows.

On the side you have tempered glass,

kind of like a Prince Rupert's Drop, but flat.

And on the windshield you have annealed glass.

First let's talk about windshields.

Glass on a windshield is designed to take a hit from a rock and not fall in on your lap.

However, glass on the side windows are different.

To demonstrate that, I got a window out of a car in a junkyard

and we're gonna hit it and just watch what happens.

Check out how it breaks.

It doesn't break.

That's because it's designed to be very very strong in the center

but not on the edge.

Watch what happens if we do the same thing on the edge.

[shatters]

The whole thing shatters.

To understand exactly why this happens

we're gonna go to my driveway and use a high speed camera

to analyze this at 20,000 frames per second.

What are you doing Mr Helper?

- Holding glass.

- He's holding the glass. This is not tempered glass.

We've got a high speed camera, this is a V1610

which at the time that we made this video is the fastest

high speed camera on the market.

And I have another helper over here.

Not you.

What are you doing?

Pushing the button when the camera breaks..

I mean when the, um..

- The camera's not gonna break! [laughs]

- The glass.

- When the glass breaks, and what does that do?

- Start the camera.

- That's right. OK.

So what we're gonna do is we're gonna drop tempered glass

and not tempered glass,

and we're gonna show the difference between the two.

And there's a special announcement at the end of this.

So, let's do some science.

You can move buddy.

- I can move?

- I've got it. Yep, safety glasses on.

You ready little girl?

Ready?

3.. 2.. 1..

[shatters]

[laughs] OK, did you do it?

[slowed shatter]

OK you ready buddy?

Tempered.

Are you ready?

Do your best.

3..2..1..

Did you get it?

It didn't break.

The tempered glass didn't break.

Dropping from higher this time.

Try to drop it on the corner too.

3.. 2.. 1..

Yahaa!

It's not breaking!

OK so it wasn't breaking so we're gonna tape this wrench onto the bottom

of the glass.

And the idea is that maybe the wrench will actually twist the edge

and break the glass when it impacts.

We'll see.

Hopefully it works.

3.. 2.. 1..

[smashes]

There we go.

[slowed smash]

OK this is the aftermath.

You can see on the left we have tempered glass.

On the right, that's just normal annealed glass

so the question is, why do they break differently?

It's pretty awesome though isn't it.

OK so what did we learn from this experiment?

Annealed glass is just normal glass that's been

heated up in the manufacturing process and then allowed to cool down gradually.

This cooling relieves any internal stresses on the glass.

You'll notice when the annealed glass breaks,

not only is the cracking slow but sometimes it doesn't even make it to the other side of the glass.

Which is good for windshields.

A small crack in one spot of the windshield doesn't completely blind your vision.

Also they put a piece of plastic between two sheets of annealed glass

so that these big shards don't fall in and cut you inside the car.

Tempered glass is very different.

Instead of allowing the glass to cool down slowly

they use forced air to cool down the outer sides of the glass really quickly

which leaves the inside of the glass hot.

Kind of like how a Prince Rupert's Drop is made if you remember that video.

The inside is still hot so as it cools down

it shrinks and pulls in which puts the outside of the glass in compression

but the inside in really high tension.

Just like the Prince Rupert's Drop if you can get through that hard outer layer

it releases the tension and the feathering front destroys the entire sheet of glass.

On the Prince Rupert's Drop you have to get through the outside curved surface

of the drop to release the tension.

But on a car window you have to get through the flat tempered surface to release the tension.

With a sheet of glass though, we have to start it along one of the edges.

Even after you break tempered glass

you can sometimes still hear the stress releasing itself as it continues to break.

So there you go.

The next time you need to escape from a car window

remember that the windshield is annealed

and it's gonna hold together

but the side windows are tempered and they're gonna bust out

completely as long as you hit them on the edge.

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Hope you enjoyed this video.

If you want to subscribe feel free to do that.

If not, I'm Destin.

You're getting Smarter Every Day.

Have a good one.

That's pretty cool isn't it.

I want to say thank you to a really awesome security guard

that decided not to stop me.

Thank you very much.

What was your name?

- Tony.

- Tony. Tony has common sense and he's awesome.

I really appreciate that.

kiwicrate.com/smarter. First link in the description. Bye.