why rocket is working on the principle of law of conservation of linear momentom

Before launch, the rocket is at rest on the launch pad, so its momentum is zero. When the rocket engines fire, burning gases are expelled from the back of the rocket. By virtue of the law of conservation of momentum, the total momentum of the rocket and fuel must remain zero. The momentum of the escaping gases is regarded as having a negative value because they travel in a direction opposite to that of the rocket's intended motion. The rocket itself, then, must have momentum equal to that of the escaping gases, but in the opposite (positive) direction. As a result, the rocket moves forward.