When you pump your bicycle air pump fifty times, the work that you do is being stored inside the pop bottle as compressed air.

Air is very springy and when it gets a chance, it bounces back to its original volume. The only way it can do that is to come rushing out the nozzle at the bottom of the rocket.

You feel tired after doing fifty pumps because you have delivered some of your energy to the rocket. But the rocket now contains the energy that you have lost, in the form of compressed air.

A lot of people make the mistake of thinking the rocket pushes on something (for instance, the ground). This is not true.

The rocket moves forward by throwing its exhaust (the air and water) out the nozzle as fast as possible.

It does not push on anything, like the ground or atmosphere. Example: if you were sitting in a small boat full of baseballs and one-by-one threw the baseballs out the back of the boat, the boat would move forward on the water.

The faster you threw the baseballs out the back of the boat, the more force would be applied to moving the boat forward.

This is not a very good way of moving your boat around.

And indeed, it is a very inefficient way of moving a rocket around, but what else can you do? There’s nothing to push on in outer space.

Antigravity water rockets work much the same way that a bullet does. Each stage of the rocket (and indeed the entire rocket!) is inherently unstable and tail-heavy. Stability and straight-up flight are achieved by spinning the rocket. When a rocket stage is out of pressure and has reached as high as it can go, the spinning slows down or stops and the tail of the rocket starts to fall first. However the tail has more air friction than the front and the falling rocket soon settles in to a gentle sideways descent.

Space tip: Outer space rockets don’t have any fins because there is no air out there to push the fins back. They use gyro-sensors to automatically aim the nozzle in different directions to keep the rocket pointed straight.

In the first fraction of a second when a rocket is lifting off of the ground, it is not going fast enough for the fins to keep it pointed straight up. And the angled fins are not moving fast enough through the air to spin the rocket. To solve this, on the base of the launcher there are three curved guide rods that engage in 6 loops on the Stage 1 ring fin structure. During liftoff, the rocket moves up and is automatically spun by the three curved rods. When the rocket leaves the curved rods it is spinning fast enough for the rocket to be very stable and go straight up.

The yellow bulb at the end of the filling hose is completely closed except for a tiny  pinhole that acts as a one-way valve. When you pump up the rocket with air, the pressure in the hose makes the bulb expand and press on the inside of the rocket nozzle to hold on to it.

The air also pushes through the pinhole to fill the rocket.

When the pressure in the rocket bottle reaches about 80 pounds per square inch, the rocket’s pressure relief valve starts to open and whistle, telling you to launch the rocket. You can do this by disconnecting the hose from the pump. If you keep pumping the extra air will just come out the whistle and you’ll be wasting your energy.

When you disconnect the pump from the filling hose, the pressure inside the bulb is reduced to ambient and the pressure in the bottle shrinks the bulb and pushes it out, launching the rocket. You can do this at any time; you don’t have to wait for the whistle.