Yeah, they're a lot of fun! (And relatively safe: you might get hit in the eye but you won't get burned or start a fire.) You need the water for reaction mass

Yeah, they're a lot of fun! (And relatively safe: you might get hit in the eye but you won't get burned or start a fire.) You need the water for reaction mass, but water isn't compressible so you need air too to have something to pressurize to store energy. https://en.wikipedia.org/wiki/Water_rocket https://en.wikipedia.org/wiki/Water_rocket My favorite toy rocket is the venerable Matchstick Rocket. You lay a pin alongside a match with the tip of the pin against the head of the match, and then wrap the top half of both in a scrap of aluminum foil. Remove the pin, leaving a hollow nozzle for the gasses to escape. Perch the rocket on a bent-paperclip launch cradle and hold another lit match under the foil-wrapped head to ignite the solid fuel and initiate liftoff. They'll fly a meter or two, once you get the kinks out of the manufacturing process. Ooo! NASA has a page on making them: https://www.grc.nasa.gov/WWW/k-12/TRC/Rockets/match_rocket.html https://www.grc.nasa.gov/WWW/k-12/TRC/Rockets/match_rocket.h...