A water rocket is a type of rocket that uses water and pressurized air as the energy to propel it out the nozzle. The bottle is filled partially with water and then pressurized with air. Pushing the water out the nozzle at the back creates thrust and causes the rocket to move in the other direction.  For the rocket to be in flight for the longest time you have to find the best ratio of water to air.  If the rocket has too much water and not enough air, then it will be heavy and won’t accelerate fast. It also won’t have enough air to thrust out all the water. When there is a lot of water in the bottle the air in the bottle needs to expand many times its original volume in order to push out all the water. As the air expands the pressure drops, when the pressure inside reaches the same pressure as outside no more water comes out. If the expansion of the air stops before all the water is pushed out, then the remaining water is extra weight which reduces the acceleration of the rocket