DENJIRO: Today's topic is glass. This glass bottle here has water in it. If I were to smash this bottle with a bat, you'd think the bottle would shatter and the water will spill everywhere. Well, let's give it a shot.
I made a clean cut on the bottle. It was actually scratched a little. This is why the top cleanly broke off. So why did that happen? Let me show you an experiment with a glass rod. First, I'm going to make a thin glass rod. I've now suspended a 2-kg weight from it. Let's have him lift it. Glass is a strong material, so even this thin rod is able to handle the weight. But watch what happens if I scratch it with sandpaper. It broke.
Even though it was a small scratch, the weight made it get bigger, and that's why the glass broke. You can break glass by scratching it and applying force to it. But you can break it using heat as well. Here's a bottle with a scratch in the middle. Let me heat it up with a gas burner. I'm going to place the bottle in water.
So why did the bottle break? Glass expands when you heat it. When you quickly cool it down, the outside immediately shrinks. This causes stress on the glass, and that's why it breaks.
If you have a special type of glass called "fused quartz," it doesn't expand or shrink from heat. As a result, fused quartz test tubes don't break as easily from heat.
I hope energy will bring you all happiness. Our magic word is "Happy Energy!"