private void Update() {
         Rigidbody rb = GetComponent();
         if (Input.GetKey(KeyCode.A))
             rb.AddForce(Vector3.left);
         if (Input.GetKey(KeyCode.D))
             rb.AddForce(Vector3.right);
         if (Input.GetKey(KeyCode.W))
             rb.AddForce(Vector3.up);
         if (Input.GetKey(KeyCode.S))
             rb.AddForce(Vector3.down);