Here's why:
* Newton's Third Law: For every action, there is an equal and opposite reaction.
* How it applies to rockets:
* Action: The rocket engine burns fuel and expels hot gas downward (the action).
* Reaction: The expelled gas pushes back on the rocket with an equal and opposite force, propelling it upward (the reaction).
Essentially, the rocket pushes the exhaust gases down, and the gases push the rocket up. This is what generates the thrust that overcomes gravity and allows the rocket to achieve lift-off.