For my science class, we are creating rockets made up of 2 liter soda bottles that are to be launched while hanging on a fishing line. We are not allowed to use chemicals or heat (fire), and liquid cannot spill out from the rocket. The rocket must be able to travel all the way across the room, which is about 15 meters in length. It also must be fastest in a race against an opposing rocket. What would be the most efficient way (and perhaps even the easiest way) to propel the rocket?