Let's learn a little more about Rockets!
One thing that all rockets need is something called propellant.
This is the fuel used to push a rocket through the air.
Propellants can be solid like gunpowder, liquid like water, gasoline or liquid oxygen, or they can be a gas, like compressed nitrogen, or just pressurized air.
When the rocket is started, the propellant shoots out of the rocket at a super fast speed, pushing it along.
So this could be just a long skinny water balloon that shoots out it's water, or it could be a fireworks bottle rocket that burns up it's powder and shoots fire out the back.
The propellant is the stuff inside the rocket that makes the rocket go.
(from: wikipedia - ammonium perchlorate composite propellant)
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