The water powered rocket illustrates three important science principles:
1) Newton’s third Law of motion,
2) Boyles Law, and
3) as conditions change so do results.
Newton’s third law of motion is basically that for every action there is an equal and opposite reaction. In the case of the water rocket, as water moves earthward the rocket moves skyward.
Boyles Law, in essence is that for a given amount of gas in an enclosed space, as the volume decreases the pressure increases. And third, that for a given system, as conditions change so do results. We start out with the bottle at rest and full of air. As water rushes in, the air in the bottle is compressed and the pressure increases. But the bottle does not move because there is friction between the mouth of the bottle and the outside of the pipe. However, as more water enters, the pressure in the bottle increases pushing some of the water out decreasing the friction. Soon the pressure in the bottle overcomes the friction holding it. As the high water pressure pushes the water out (toward the ground) the bottle moves in the opposite direction (toward the sky).
We have lift off!