Water rockets are fun toys for people of all ages, even adults. They work based on a simple physics concept in which water is forced out the bottom of the rocket by air pressure, and in doing so exerts an upward force pushing up on the rocket (Newton’s third law). This upward force causes the rocket to shoot upward at high speed. The way a water rocket works is by filling it up partially with water and then pressurizing the inside with air. When the bottom nozzle is opened the internal air pressure forces the water out of this nozzle at high speed causing the rocket to shoot straight up at high speed.
Water rocket enthusiasts have created all sorts of amazing water rocket designs, including this one shown in the video below which is a two-stage water rocket.