The Secret of Tempered Glass / Mr. Denjiro's Happy Energy!

DENJIRO: I have a glass bottle here. Let's break it by lightly tapping on it. Watch.
Let me tell you how I did it. When I tapped on the bottle, I dropped in a small rock, which is harder than the glass. This bottle is called a "Bologna bottle," and it's undergone special processing. Let's look at the bottle through a polarizing plate. It looks like the bottle is colored. This is proof that there is tension in the bottle. This is the key to making the Bologna bottle.
Let me explain using this glass stick. First, we heat the tip and form a ball. When the ball cools down, it shrinks as well. Let's look at it through the polarizing plate. Since we cooled it down slowly, the outside and inside of the ball cooled down at the same pace and hardened, so there's no tension in the glass. That's why it looks transparent.
However, when we use wind to cool down the outside quickly… the outside hardens fast, but the inside shrinks and hardens slowly. This causes tension within the glass.
NARRATOR: The Bologna Bottle is also made by quickly cooling the outside. This creates tension within the bottle, causing the outside to be strong but the inside to be fragile. That's why the bottle broke when the rock scratched the inside.
DENJIRO: This principle is also used in tempered glass. Tempered glass is created by heating the glass up to over 650 degrees Celsius, and then rapidly cooling the outside to create a strong surface. So, even when I hit the glass like this, it doesn't break. However, there is tension inside the glass, so when I strike it with this hammer with a sharp tip…
This hammer is used for escaping a car during an emergency. The sharp tip shaves down the hard surface and damages the weak inside of the glass. That's why the glass easily shattered into pieces.
I hope energy will bring you all happiness. Our magic word is "Happy Energy!"