Note that this is a good story for the skeptics to keep handy. It seems to be a good example of how easily people can be fooled.
To make hydrogen you mix some lye, carefully, with water. When it cools enough, you start tearing off pieces of aluminum foil and throwing them in the lye water. They start to bubble furiously, and the bubbles are hydrogen. If you do all this in a bottle, it's very easy to fill a small garbage bag with the gas by just pinching the opening of the bag around the bottle's mouth.
We used to do this all the time when I was kid. We made our own fuses from tissue paper dipped in wet saltpeter and sugar. When these dried, we would tape them to a bag of hydrogen, light the fuse, then let them float up and away. In a little bit there would be a nice, satisfying fireball.
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