When a rocket lifts off, the upward momentum it gains is matched by the downward momentum of the exhaust gases it expels. This is due to the principle of conservation of momentum.

Here's a breakdown:

* Momentum: Momentum is a measure of an object's mass in motion. It's calculated as mass (m) multiplied by velocity (v): Momentum (p) = m * v

* Conservation of Momentum: This principle states that the total momentum of a closed system remains constant. In other words, momentum cannot be created or destroyed, only transferred.

* Rocket Launch: When a rocket burns fuel, the combustion creates hot, expanding gases. These gases are expelled out the rocket nozzle at high velocity. This downward momentum of the exhaust gases is equal and opposite to the upward momentum gained by the rocket.

Think of it this way: The rocket pushes the exhaust gases downwards, and in turn, the exhaust gases push the rocket upwards. This is how the rocket gains enough momentum to overcome gravity and launch into space.