How does bottle rockets fly?

Answer 1

You throw them up the air.

There is two ways to explain a bottle-rocket's

drive - by accelerating forces and by the law of conservation of momentum.

The bottle is pressurised and pressure pushes equally in all directions. Therefore there is an equal force on the inside of the bottle in all directions except at the neck, where there is a hole instead of bottle. The force on the open top is therefore less than the force on the bottom, so the bottle accelerates - bottom first.

The other way of explaining is to realise that the bottle is hurling water out of the open neck and therefore there is a recoil force on the bottle in the opposite direction.