it says an intance of type "unityEngine.rigidbody" is requrid to access non static member addrelativetorque. what does this mean??\

#pragma strict var rotationSpeed = 100; function Update () { var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed; rotation *= Time.deltaTime; Rigidbody.AddRelativeTorque (Vector3.back * rotation) ;