void FixedUpdate ()
    {
        horizontalMoving = Input.GetAxis("Horizontal");
        verticalMoving = Input.GetAxis("Vertical");

        Vector3 movement = new Vector3 (horizontalMoving, 0.0f, verticalMoving);
        rb.velocity = movement * speed;

        rb.position = new Vector3
            (
                Mathf.Clamp(rb.position.x, boundary.xMin, boundary.xMax),
                0.0f,
                Mathf.Clamp(rb.position.z, boundary.zMin, boundary.zMax)
            );

        rb.rotation = Quaternion.Euler (0.0f, 0.0f, rb.velocity.x * -tilt);
    }