Combustion of Hydrogen / Mr. Denjiro's Happy Energy!

DENJIRO: Everyone, we have a candle burning here, but do you know what exactly is burning here?
BOY: The wax is burning, right?
DENJIRO: What’s burning here is the carbon and hydrogen that is present in the wax. Today, I’ll show you an interesting phenomenon regarding combustion of hydrogen.
I have here a bag filled with hydrogen. First, I’m going to inflate a soap bubble using only hydrogen.
I’m going to put a flame up to this. The hydrogen caught fire.
Now I have another bag. This has air mixed in the hydrogen. I’m going to inflate a soap bubble using this. What will happen if I put a flame up to it?
BOY: That’s a strong reaction!
DENJIRO: As you can see, hydrogen burns violently when mixed with air. Now, let me use this property of hydrogen to show you an even more surprising experiment. This transparent tube has two lids on it. They both have a hole in them. I’m going to fill the tube with water. Now that the tube is full of water, I’m going to close the hole on the top and put hydrogen into the tube from the hole in the bottom. By doing this underwater, the water in the tube is replaced with hydrogen.
Now, I’ve taken the tube out of the water and stood it upright. Let me put a flame up to it. The hydrogen coming out of the tube is burning. Hydrogen flame is hard to see, but we’ve made it easier to see by putting salt around the hole to give the flame a color. Watch what happens next.
BOY: What a surprise!
DENJIRO: A reaction occurred a while after we set fire to it. Why did this happen? When the hydrogen coming out of the hole on top is burned, air enters through the hole in the bottom. As the amount of hydrogen coming out decreases, the flame reacts with the mixture of hydrogen and air inside the tube. Therefore, we got a strong reaction like we saw.
I hope energy will bring you all happiness. Our magic word is "Happy Energy!"