Tis the season to be providing your kids with fun toys, hopefully of the type that foster an interest in science and technology. Also without your having to polish your negotiating skills with the local constabulary. In that spirit, here's the method for turning a 2-liter pop bottle into a fun demonstration of propulsive science.
The easy method is to start with a clean, dry 2-liter bottle, with the cap. Drill a hole in the cap. Diameter is not terribly important, but 1/4 at a minimum, and 3/8" at a maximum seem to work best.
Get the Isopropyl alcohol (IPA), and pour 1/4 to 1/3 of a capful into the bottle. That's capful, not cupful. screw on the cap, and shake the bottle a few times.
Lay the bottle on a smooth surface, pointing in a safe direction. Using a lighter, applied from the side (hot exhaust!) hold the flame to the hole in the cap until the vapors inside the bottle ignite.
With a loud whoosh, the bottle will propel itself up to 30 feet downrange. Squeeze the spent bottle a few times to refresh the air inside, and attempt to re-ignite. Spend the next 15 minutes arguing weather the initial shot was too rich or too lean. Refuel and repeat.
For you wannabe rocket scientists with access to machine tools, enlarge the hole in the cap to .81" and make a real rocket nozzle:
The bottle cap goes on over the right end. Debate endlessly weather this actually makes any difference to the flight characteristics.
Use IPA and air only and you won't damage the bottle beyond some wrinkling caused by the heat. Eventually you'll want to replace the bottle with one that still has a straight center axis. We have tried other flammable liquids but not noted any difference in performance. IPA is easy to come by, and easy to handle.
Builders assume all risks, so if you figure out how to blow this up, don't come crying to me. Remember: IPA and air only.