Rockets change their velocity using a principle called Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.

Here's how it works:

* Rocket Engines Burn Fuel: A rocket engine burns fuel, producing hot, expanding gases.

* Gases Expel Out the Back: These gases are forcefully expelled out the back of the rocket through a nozzle.

* Reactionary Force: As the gases are expelled, they push against the rocket in the opposite direction, creating a force known as thrust.

* Acceleration: This thrust force causes the rocket to accelerate, meaning it changes its velocity.

To summarize, a rocket changes its velocity by expelling mass (hot gas) in the opposite direction of the desired motion.

Here are some additional points:

* Direction of Thrust: By changing the direction of the thrust, the rocket can change its direction of motion.

* Amount of Thrust: The amount of thrust determines how quickly the rocket accelerates. More thrust means faster acceleration.

* Rocket Stages: To achieve higher speeds, rockets often use multiple stages. Each stage is a self-contained unit that provides thrust for a period of time and then separates from the rocket. This allows the rocket to shed weight as it burns fuel.

In simple terms, rockets work by pushing against something (the expanding gas) to move forward.