Have you ever wanted to fire a water bottle from a shoulder-mounted launcher? I know I have. This Instructable takes the basic "water bottle rocket" science lesson and expands on it, combining the ability to aim with a convenient trigger mechanism. It can be built in a few hours using parts from a hardware store. But before I continue, here is the requisite disclaimer:
PLEASE, do not aim the Bazooka at people, animals, your neighbour's prize-winning tomato plants, or other objects that may be damaged. Only launch outside. Don't do anything stupid, like lighting the bottle on fire before launching it. Use only water as a propellant. Do not over-pressurize the bottle. Use under adult supervision. Keep small children away (regardless of my video!) Use the thinking part of your brain.
MOST IMPORTANTLY, do try to retrieve every bottle you launch. Let's not litter, okay? I'd also suggest using "reclaimed" water as propellant, such as rainwater, snow melt, or the like.