ELI5: Explain Like I'm 5

rocket engines

A rocket engine is like a giant balloon that has a lot of air inside that is on fire. When the air is on fire, it makes a lot of pressure inside the balloon which makes the whole rocket move really fast. It's like blowing on a balloon to make it move - the air coming out makes the balloon move away from you. In the rocket engine, the air is coming out much faster, so it makes the whole rocket move!