Classical mechanics
A rocket works because of Newton's third law of motion: the bottom of the rocket makes very hot gas and the gas pushes the colder air. Then the rocket goes up because the colder air also pushes the bottom of the rocket. The force that makes a rocket go up is called thrust. Birds and airplanes fly because of Newton's third law of motion: this is because both birds and airplanes push air down when they move, and the air pushes them up. This force is called lift. Without lift, birds and airplanes fall on the ground.