Once you have the RPG-7 built, you fill a plastic bottle with water, set the launcher, pump air into the launcher (with a bicycle pump), and pull the trigger. Then, whoosh! The plastic bottle flies through the air, leaving a trail of water. The launcher even has a realistic recoil.
Note that the kit includes plastic bottle conversion parts to turn beverage containers into green, pointy MISSILES.
The PET Bottle Launcher RPG-7 is aimed at adults and is 18 and up. When I say "aimed at adults", I do not mean "aimed at adults."
R・P・G！ あのロケットランチャーを再現したペットボトルランチャー「RPG-7」が発売中 [Kotaku Japan]