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Newton's third law states that every action has an equal and opposite reaction. When a helicopter's propeller spins, the helicopter body will follow this law, and try to spin in the opposite direction! This is what we call torque. 

To stop this from happening, we need to find a way to conserve the momentum of the propeller. This is why we use a tail rotor, which works to fight against torque. This is why, in many Hollywood movies, when the tail is destroyed, the helicopter spirals out of control.