We've been fiddling around with an electrolyser that we made, converting water into a gaseous hydrogen/oxygen mixture. After producing about half of a large juice container full, we were ready to test our "rocket".
The igniter is just an extension cord, cut off at one end, and with two tiny inner wires connected to make a short circuit (and a spark). You can see this at the very beginning of the video clip.
So anyway, the cord and igniter was shoved into the bottle, and it was flipped over into a bowl of water, so as not to let any of the gas escape. It worked pretty well, too, considering that this was the first time we've actually tried this (other than just blowing up bags of hydrogen ).
We got around three metres of height. Enjoy the video I'll keep you guys posted as we build more prototypes.
P.S. I'm the guy who says "woah" after it goes off, in case anyone cares...
EDIT: By the way, the reason we sound so excited is because we totally didn't expect it to work at all. This was the first time we had done it, and the hypothesis was that the top of the bottle would be too weak, and would just burst under load of the explosion.