Four dads with kids from older classes supplied the launchers, but anybody could join in the fun. The launchers ranged from simplicity itself (mere tape supplying the friction to hold the rocket back while air was pumped in) to the elaborate (a finely-crafted wooden unit with separate metal braces for releasing after you had pressurized the bottle). You could experiment with putting various amounts of water in the bottle. Some people had bottles outfitted with fins and nosecones. However, we found that bigger was better, and that 2-liter bottles (of which we brought the first) launched impressively even without modification.
And it goes a little something like this (MPEG-4/QuickTime 6, 1.3 MB).